Fix jruby CI builds

For jruby we need to stick with rubygems 2.7.4 until
rubygems/rubygems#2188
is fixed and released.

Without this workaround, for jruby builds, rubygems
activates jruby stdlib minitest (v5.4.1) instead of the
bundled version (v5.11.3).

.travis.yml

@@ -10,8 +10,14 @@ rvm:
   - jruby-9.1.16.0
 
 before_install:
-  - gem update --system
-  - gem install bundler
+  # For jruby we need to stick with rubygems 2.7.4 until
+  # https://github.com/rubygems/rubygems/issues/2188
+  # is fixed and released.
+  #
+  # Without this workaround, for jruby builds, rubygems
+  # activates jruby stdlib minitest (v5.4.1) instead of the
+  # bundled version (v5.11.3).
+  - if [ "$TRAVIS_RUBY_VERSION" = "jruby-9.1.16.0" ]; then gem update --system 2.7.4; else gem update --system; fi
 
 gemfile:
   - gemfiles/rails_5_2.gemfile