.travis.yml: add Travis-CI script

Add a simple .travis.yml file that shells out testing to the test.bash script.
Caching the toolchain directories (Go toolchain and protobuf toolchain) saves
around 14min on each run.

This script only runs after Gerrit changes have been pushed to GitHub and acts
as a form of post-submit tests.

Change-Id: Id7d73d2c5fdc0de259d93cceb341ce921f71951d
Reviewed-on: https://go-review.googlesource.com/127915
Reviewed-by: Herbie Ong <herbie@google.com>

.travis.yml

@@ -0,0 +1,5 @@
+script:
+  - ./test.bash
+cache:
+  directories:
+  - /tmp/golang-protobuf-test