Lock rubygems version to 2.7.9

deivid-rodriguez · deivid-rodriguez · commit 0d7c06d00333 · 2019-03-15T08:54:47.000+01:00
Since it's the last version compatible with the full current matrix.
diff --git a/.travis.yml b/.travis.yml
@@ -1,5 +1,5 @@
 before_install:
-  - gem update --system
+  - gem update --system 2.7.9
   - gem install bundler
   - rvm @global do gem uninstall did_you_mean
 bundler_args: --without development
