Merge pull request activeadmin#149 from activeadmin/ci-ruby-2-7

Run travis for ruby 2.7

Author: grzegorz-jakubiak

.travis.yml

@@ -3,9 +3,11 @@ services:
   - mongodb
 before_install:
   - gem update --system
-  - gem install bundler   
+  - gem install bundler
+  - bundle update --bundler
 rvm:
   - 2.3
   - 2.4
   - 2.5
   - 2.6  
+  - 2.7