Limit branch builds to master. (#50)

.travis.yml

@@ -3,8 +3,9 @@ services:
   - docker
 before_install:
   - docker build -t bcbc/workstand:production .
-after_success:
-
+branches:
+  only:
+  - master
 after_success:
   - if [ "$TRAVIS_BRANCH" == "master" ]; then
     docker login -u="$DOCKER_USERNAME" -p="$DOCKER_PASSWORD";