Force 1.8.25 rubygems version

The rvm command is not working, as we can see this output from travis:
$ rvm rubygems 1.8.25
Installed rubygems 2.2.2 is newer than 1.8.25 provided with installed ruby, skipping installation, use --force to force installation.

We can use the gem --system command to install a older version
Merge pull request #684 from arthurnn/fix_travis

Force 1.8.25 rubygems version