Workaround for bundler issue

Breaking with 'You have already activated bundler 1.17.2, but your Gemfile requires bundler 1.17.3...'

.travis.yml

@@ -4,6 +4,7 @@ before_install:
   # https://github.com/danmayer/coverband/issues/162#issuecomment-452173268
   - rm /home/travis/.rvm/gems/*/specifications/bundler-2.*.gemspec || true
   - gem install bundler -v '< 2'
+  - gem update --system
 
 before_script:
   - cd spec/dummy_app