workaround for gem installation on RHEL/CentOS

On at least RHEL/CentOS 6.3, the "gem update --system" results in a v2.0.0 gem installation.
This breaks dependencies with rubygems already installed as part of chef-service.

This is a temporary workaround to install the highest pre-2.0.0 version, namely 1.8.25.

Issue on Cloudify Jira:
https://cloudifysource.atlassian.net/browse/CLOUDIFY-1543