Update Ubuntu (especially updating "latest" to point to "xenial")

- `ubuntu:precise`: 20160425
- `ubuntu:trusty`: 20160424
- `ubuntu:wily`: 20160424
- `ubuntu:xenial`: 20160422

library/ubuntu

@@ -5,31 +5,31 @@
 # see also https://wiki.ubuntu.com/Releases#Current
 
 # commits: (master..dist)
-#  - 028503f Update tarballs
-#    - `ubuntu:precise`: 20160330
-#    - `ubuntu:trusty`: 20160412
-#    - `ubuntu:wily`: 20160329
-#    - `ubuntu:xenial`: 20160331.1
+#  - 0aeadd4 Update tarballs
+#    - `ubuntu:precise`: 20160425
+#    - `ubuntu:trusty`: 20160424
+#    - `ubuntu:wily`: 20160424
+#    - `ubuntu:xenial`: 20160422
 
-# 20160330
-12.04.5: git://github.com/tianon/docker-brew-ubuntu-core@028503f527c77921a3eff88216f57025cc611869 precise
-12.04: git://github.com/tianon/docker-brew-ubuntu-core@028503f527c77921a3eff88216f57025cc611869 precise
-precise-20160330: git://github.com/tianon/docker-brew-ubuntu-core@028503f527c77921a3eff88216f57025cc611869 precise
-precise: git://github.com/tianon/docker-brew-ubuntu-core@028503f527c77921a3eff88216f57025cc611869 precise
+# 20160425
+12.04.5: git://github.com/tianon/docker-brew-ubuntu-core@0aeadd4fc54b3624e147e8e11896bf66ddba79ce precise
+12.04: git://github.com/tianon/docker-brew-ubuntu-core@0aeadd4fc54b3624e147e8e11896bf66ddba79ce precise
+precise-20160425: git://github.com/tianon/docker-brew-ubuntu-core@0aeadd4fc54b3624e147e8e11896bf66ddba79ce precise
+precise: git://github.com/tianon/docker-brew-ubuntu-core@0aeadd4fc54b3624e147e8e11896bf66ddba79ce precise
 
-# 20160412
-14.04.4: git://github.com/tianon/docker-brew-ubuntu-core@028503f527c77921a3eff88216f57025cc611869 trusty
-14.04: git://github.com/tianon/docker-brew-ubuntu-core@028503f527c77921a3eff88216f57025cc611869 trusty
-trusty-20160412: git://github.com/tianon/docker-brew-ubuntu-core@028503f527c77921a3eff88216f57025cc611869 trusty
-trusty: git://github.com/tianon/docker-brew-ubuntu-core@028503f527c77921a3eff88216f57025cc611869 trusty
-latest: git://github.com/tianon/docker-brew-ubuntu-core@028503f527c77921a3eff88216f57025cc611869 trusty
+# 20160424
+14.04.4: git://github.com/tianon/docker-brew-ubuntu-core@0aeadd4fc54b3624e147e8e11896bf66ddba79ce trusty
+14.04: git://github.com/tianon/docker-brew-ubuntu-core@0aeadd4fc54b3624e147e8e11896bf66ddba79ce trusty
+trusty-20160424: git://github.com/tianon/docker-brew-ubuntu-core@0aeadd4fc54b3624e147e8e11896bf66ddba79ce trusty
+trusty: git://github.com/tianon/docker-brew-ubuntu-core@0aeadd4fc54b3624e147e8e11896bf66ddba79ce trusty
 
-# 20160329
-15.10: git://github.com/tianon/docker-brew-ubuntu-core@028503f527c77921a3eff88216f57025cc611869 wily
-wily-20160329: git://github.com/tianon/docker-brew-ubuntu-core@028503f527c77921a3eff88216f57025cc611869 wily
-wily: git://github.com/tianon/docker-brew-ubuntu-core@028503f527c77921a3eff88216f57025cc611869 wily
+# 20160424
+15.10: git://github.com/tianon/docker-brew-ubuntu-core@0aeadd4fc54b3624e147e8e11896bf66ddba79ce wily
+wily-20160424: git://github.com/tianon/docker-brew-ubuntu-core@0aeadd4fc54b3624e147e8e11896bf66ddba79ce wily
+wily: git://github.com/tianon/docker-brew-ubuntu-core@0aeadd4fc54b3624e147e8e11896bf66ddba79ce wily
 
-# 20160331.1
-16.04: git://github.com/tianon/docker-brew-ubuntu-core@028503f527c77921a3eff88216f57025cc611869 xenial
-xenial-20160331.1: git://github.com/tianon/docker-brew-ubuntu-core@028503f527c77921a3eff88216f57025cc611869 xenial
-xenial: git://github.com/tianon/docker-brew-ubuntu-core@028503f527c77921a3eff88216f57025cc611869 xenial
+# 20160422
+16.04: git://github.com/tianon/docker-brew-ubuntu-core@0aeadd4fc54b3624e147e8e11896bf66ddba79ce xenial
+xenial-20160422: git://github.com/tianon/docker-brew-ubuntu-core@0aeadd4fc54b3624e147e8e11896bf66ddba79ce xenial
+xenial: git://github.com/tianon/docker-brew-ubuntu-core@0aeadd4fc54b3624e147e8e11896bf66ddba79ce xenial
+latest: git://github.com/tianon/docker-brew-ubuntu-core@0aeadd4fc54b3624e147e8e11896bf66ddba79ce xenial