Report error when unsupported Overcommit installed

For users that are already running an older version of Overcommit,
they'll get a weird error if they somehow install the new hooks on their
machine and attempt to run them.

This solves this problem by reporting a helpful error message if the
version of overcommit that was loaded is less than a particular version.

Change-Id: I6fa26c05e0224daffc0d0909bff0d11b91506094
Reviewed-on: http://gerrit.causes.com/35621
Reviewed-by: Shane da Silva <shane@causes.com>
Tested-by: Shane da Silva <shane@causes.com>

Author: sds

template-dir/hooks/overcommit-hook

@@ -40,6 +40,14 @@ rescue LoadError
   end
 end
 
+if Gem::Version.new(Overcommit::VERSION) < Gem::Version.new('0.6.0')
+  puts "Installed version of Overcommit (#{Overcommit::VERSION}) is " <<
+       "incompatible with the installed hooks.\n" <<
+       'Run `gem install overcommit && overcommit --install` to ensure ' <<
+       "you're up-to-date."
+  exit 64 # EX_USAGE
+end
+
 begin
   hook_type_class = Overcommit::Utils.camel_case(hook_type)