Update install.rst

Added text and link to pre-release version of Compass under 'Compass' section of Install.

Users of version < 1.0 of Compass will encounter the following error when installing: 'No such framework: "susy"'
This is a great idea, but I'm not sure how this link helps. The beta compass docs don't say anything about how to install a pre-release. I think we need to be more clear.