Pin to an older Rubygems gem on Travis

Author: sodabrew

.travis.yml

@@ -2,7 +2,9 @@ language: ruby
 bundler_args: --without benchmarks
 script:
   - bundle exec rake spec
+# Temporary workaround for broken Rubygems on Travis
 before_install:
+  - gem update --system 2.4.8
   - gem --version
   - |
     bash -c " # Install MySQL 5.7 if DB=mysql57