Don't install prerelease gems, they're way out of date

there hasn't been a prerelease version since 1.5.0.pre.1657 on
2013-12-27.  The latest is 1.2624.0, released 2014-07-01.
Merge pull request #23 from philandstuff/remove-pre-switch

Don't install prerelease gems, they're way out of date