add danger; build in venv

.travis.yml

@@ -19,6 +19,10 @@ cache:
     - $HOME/Library/Caches/pip
     - $TRAVIS_BUILD_DIR/cache/wheel
   
+before_install:
+  - virtualenv venv
+  - source venv/bin/activate
+
 install:
   - pip install -U pip
   - pip install -r requirements.txt
@@ -28,3 +32,4 @@ script:
   - ./run_pylint.sh
   - pip install mock trial
   - ./run_tests.sh
+  - rvm use 2.3.1 && gem install danger --version '~> 4.0' && danger; fi