Set up default Rake task, which runs specs

This is principally for the benefit of Travis CI, which will run the
default task in order to perform a build.

Change-Id: Ie60d0694a64e7b6013c1f2d2248485c5f387cb8f
Reviewed-on: https://gerrit.causes.com/26294
Reviewed-by: Lann Martin <lann@causes.com>
Tested-by: Greg Hurrell <greg@causes.com>