ensure branch and set cache to discourse/vendor

.travis.yml

@@ -6,7 +6,7 @@ env:
 
 dist: trusty
 
-cache: bundler
+cache: bundler --path $TRAVIS_BUILD_DIR/discourse/vendor
 
 addons:
   postgresql: 9.5
@@ -29,7 +29,7 @@ before_install:
 
 install:
   - touch config/stripe.yml
-  - git clone $TRAVIS_BUILD_DIR plugins/discourse-payments --depth 50
+  - git clone $TRAVIS_BUILD_DIR plugins/discourse-payments --depth 50 --branch $TRAVIS_BRANCH
 
 before_script:
   - gem install bundler