.travis.yml in onkcop-0.52.1.1 vs .travis.yml in onkcop-0.53.0.0

@@ -1,8 +1,5 @@
 language: ruby
-cache: bundler
+# NOTE: travis build fails when `install.bunler` with cache.
+# cache: bundler
 rvm:
   - ruby-head
-# NOTE: travis build fails on bundler v1.16.1 && rubygems v2.7.3.
-# Update travis's rubygems to v2.7.4.
-before_install:
-  - gem update --system