Force travis to use bundler 2.0

Author: mlitwiniuk

.travis.yml

@@ -12,6 +12,8 @@ services:
 env:
   - LIT_STORAGE=hash
   - LIT_STORAGE=redis
+before_install:
+  - gem install bundler
 before_script:
   - cp test/dummy/config/database.yml.travis test/dummy/config/database.yml
   - psql -c 'create database lit_test;' -U postgres