Stick with a passing version of Rubygems and bundler

Rubygems 2.7.5 has a bug with JRuby and Bundler is being unstable
latelly so it is better to stick with a version we know tests are going
to pass.

.travis.yml

@@ -14,7 +14,7 @@ addons:
       - libfcgi-dev
 
 before_install:
-  - gem env version | grep '^\(2\|1.\(8\|9\|[0-9][0-9]\)\)' || gem update --system
+  - gem update --system 2.7.4
   - gem list -i bundler || gem install bundler
 
 script: bundle exec rake ci