Travis fix as suggested by @jamesonjlee (google#230)

Author: sbarzowski

.travis.yml

@@ -8,6 +8,7 @@ before_install:
   - go get github.com/axw/gocov/gocov
   - go get github.com/mattn/goveralls
   - if ! go get github.com/golang/tools/cmd/cover; then go get golang.org/x/tools/cmd/cover; fi
+  - go get github.com/sergi/go-diff/diffmatchpatch
 script:
   - $HOME/gopath/bin/goveralls -service=travis-ci
   - ./tests.sh --skip-go-test