Fix Travis caching picking up bundle changes

Because Go builds switch directories after checkout, the local
`vendor/bundle` location to which Bundler installs gems will not be
picked up by Travis caching mechanism.

As a workaround, set an absolute bundle location that sticks to the
original directory that cache is aware of.

travis-ci/travis-ci#3055

.travis.yml

@@ -8,7 +8,7 @@ language: go
 go:
   - 1.4
 script: script/test
-install: bundle install --without development --deployment --jobs=3 --retry=3
+install: bundle install --path "$HOME/build/$TRAVIS_REPO_SLUG/vendor/bundle" --without development --deployment --jobs=3 --retry=3
 cache:
   directories:
     - vendor/bundle