Update .travis.yml to account for Travis changes (especially now that Docker is no longer experimental :tada:)

.travis.yml

@@ -1,7 +1,5 @@
 language: go
 
-sudo: false
-
 install:
   - mkdir -p "${GOPATH%%:*}/src/github.com/shurcooL/markdownfmt"
   - git clone https://github.com/tianon/markdownfmt.git "${GOPATH%%:*}/src/github.com/shurcooL/markdownfmt"