Don't install prerelease gems, they're way out of date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.
add some extra libs I found I needed

I couldn't install bosh_cli and bosh_cli_plugin_micro without libpq-dev
and libmysqlclient-dev.
Merge pull request #23 from philandstuff/remove-pre-switch

Don't install prerelease gems, they're way out of date
Merge pull request #24 from philandstuff/add-libs

add some extra libs I found I needed
Set a default pair of NTP servers for convenience.
Merge pull request #25 from programsam/master

Set a default pair of NTP servers for convenience.