Specify rexml versioning

[elbartostrikesagain]

rexml is a dependency and recently introduced a breaking change by requiring gcc be installed. This gem allows any version of rexml to be installed so that breaking change happens no matter what version they use. The actual version of rexml is being sorted out here hopefully: ruby/rexml#131

I would recommend this project do 2 things based on what happens with the rexml version:

1. Set a version on rexml so that it is no longer breaking - either 3.2.6 or ~> 3.2 or ~>3 based on what they do.
2. Release a more major version update to this project that includes the rexml version that needs gcc. List gcc as a requirement in the Readme.

Thanks, kind of just waiting to see what rexml does!