Add Travis badge to README and disable cache use

The cache takes about as long to copy into the build system as cloning the repos takes anyhow, so the benefits of using it are dubious at best.

.travis.yml

@@ -3,11 +3,6 @@ language: bash
 # allow for use of Docker-based workers
 sudo: false
 
-# cache those huge source repos, if possible (since build.sh handles existing repos intelligently)
-cache:
-    directories:
-        - bashbrew/src
-
 # --no-build because we has no Docker in Travis :)
 script:
     - ./bashbrew/build.sh --no-build --all

README.md

@@ -1,5 +1,7 @@
 # Docker Official Images
 
+[![Build Status](https://travis-ci.org/docker-library/official-images.svg?branch=master)](https://travis-ci.org/docker-library/official-images)
+
 ## Contributing to the standard library
 
 Thank you for your interest in the Docker official images project! We