Bump test go to 1.7.5 (#127)

* Bump test go to 1.7.5
* Use travis container infrastructure
* Don't run the tests twice on PRs

.travis.yml

@@ -1,8 +1,9 @@
+sudo: false
 language: go
 go:
   - 1.5.4
   - 1.6.4
-  - 1.7.4
+  - 1.7.5
   - tip
 matrix:
   allow_failures:
@@ -14,5 +15,7 @@ before_install:
   - go get github.com/axw/gocov/gocov
   - go get github.com/mattn/goveralls
   - if ! go get code.google.com/p/go.tools/cmd/cover; then go get golang.org/x/tools/cmd/cover; fi
+branches:
+  only: [master]
 after_success:
   - $HOME/gopath/bin/goveralls -service=travis-ci