Use Rubygems 2.7.8 (avoid Rubygems 3) (rspec#2051)

Author: greysteil

.travis.yml

@@ -23,7 +23,7 @@ cache: bundler
 bundler_args: "--binstubs --path ../bundle --retry=3 --jobs=3"
 
 before_install:
-  - gem update --system
+  - gem update --system 2.7.8
   - gem install bundler
   - script/clone_all_rspec_repos