Adding missing closing bracket. Fixes #511
Merge pull request #512 from drs-m/master

Adding missing closing bracket.

Fixes #511: passenger_nginx package cannot vulcanize.
Added a task for cleaning up old kernels.
Preparing the 2.12.0 release.
Fixed #513: Ubuntu 12.04.4 x64 is no longer a valid image.
Preparing the 2.12.1 release.
Re-use established SSH connections.

When I introduced the mutex here to prevent threading issues, I pulled the connection out of the mutex so connections to multiple servers could be established at the same time.  This had the unintended consequence of establishing SSH connections repeatedly to the same server.  I'd still like to fix this to allow multiple servers to be connected to in parallel, but reducing single server connections from N to (N - 1) easily makes up for the lack of efficiency in the interim.  This may also fix problems with establishing a bunch of SSH connections that we never clean up properly.
Added a way to retrieve the OS version for an instance.
Added a way of specifying packages to be installed by OS version.
Omit the @os_version and @capistrano ivars when converting an InstanceItem to a hash.
Support fetching the OS version even when Capistrano is unavailable, if we can.