Fix dependency for stdlib for 'downcase'

Downcase has been added to puppetlabs/puppetdb, this patch changes the
dependency for stdlib to >= 2.2.0 as that was when downcase was first
included.

Signed-off-by: Ken Barber <ken@bob.sh>
Merge pull request #70 from kbarber/fix_stdlib_dependency

Fix dependency for stdlib for 'downcase'