chore(ci): Set bundle version for ruby 2.4.1

Author: kattrali

.travis.yml

@@ -51,7 +51,9 @@ jobs:
     - GEMSETS="test sidekiq"
     rvm: 2.4.1
   - stage: test
-    env: GEMSETS="test rubocop"
+    env:
+    - BUNDLE_VERSION=1.12.0
+    - GEMSETS="test rubocop"
     rvm: 2.4.1
     script: bundle exec ./bin/rubocop lib/
   - stage: deploy