fix(.travis.yml): run the docker-build target on all branches

This commit allows us to test dockerfile changes in PRs

Author: Aaron Schlesinger

.travis.yml

@@ -11,7 +11,7 @@ sudo: required
 install:
   - make bootstrap
 script:
-  - make test
+  - make test build docker-build
 deploy:
   provider: script
   script: _scripts/deploy.sh