Add CentOS 7 support for mesosphere install

We're using this cookbook in production at Brigade, but unfortunately we
are using `node[:mesos][:type] = 'source' because we are on CentOS 7 and
the mesophere build of mesos for CentOS 6.5 understandably doesn't work
for CentOS 7.

This adds a simple conditional which swaps out the URL to the mesosphere
repo based on the version of CentOS.
Merge pull request #25 from brigade/add_centos_7

Add CentOS 7 support for mesosphere install