Downgrade RubyGems to 2.1.11 in 1.8 mode

This issue is fixed by 2.2.1, but until Travis VM is upgraded to use it,
we need this hack.

Author: BanzaiMan

.travis.yml

@@ -2,6 +2,8 @@ language: ruby
 bundler_args: --without development
 script: bundle exec rake test_$DB
 before_script: export JRUBY_OPTS="--server -Xcext.enabled=false -Xcompile.invokedynamic=false"
+before_install:
+  - 'ruby -e "puts RUBY_VERSION" | grep ^1\.8 && gem update --system -v 2.1.11 || true'
 rvm:
   - jruby
 gemfile: