Use opscenter package instead of opscenter-free.

These packages were merged together after OpsCenter 4.0 was released:
http://www.datastax.com/documentation/opscenter/5.0/opsc/release_notes/opscReleaseNotes40_r.html
Merge pull request #129 from sethrosenblum/opscenter_package_update

Use opscenter package instead of opscenter-free.