Fallback to older rubygems for staled Ruby

Looks like rubygems >= 3.0.0 no longer support staled Ruby < 2.3.0.

Author: kamipo

.travis.yml

@@ -6,7 +6,7 @@ bundler_args: --without benchmarks development
 # Pin Rubygems to a working version. Sometimes it breaks upstream. Update now and then.
 before_install:
   - gem --version
-  - gem update --system --quiet
+  - gem update --system --quiet || gem update --system 2.7.10 --quiet
   - gem update bundler
   - gem --version
   - bash .travis_setup.sh