(maint) Require date in gemspec

The `gemspec` file uses `Date.today`, so it needs to require the `date` module.
Add Solaris 11 as a POSIX operating system

For some reason, Vagrant considers Solaris 11 as an entirely separate operating
system from Solaris 10. This patch adds `solaris11` to the list of operating
systems with POSIX-like host management in `/etc/hosts`.
Update CHANGELOG and version.rb for 2.2.1