gist: re-add 'jist' gem dependency with :requires_gem

Oops, forgetting this meant that 'gist' command would error out with
ugly messages rather than prompting the user to 'install-command'

lib/pry/commands/gist.rb

@@ -3,6 +3,7 @@ class Pry
     match 'gist'
     group 'Misc'
     description 'Playback a string variable or a method or a file as input.'
+    command_options :requires_gem => "jist"
 
     banner <<-'BANNER'
       Usage: gist [OPTIONS] [--help]

spec/commands/gist_spec.rb

@@ -2,10 +2,14 @@
 # THey need to be updated for the new 'gist' API, but im too sleepy to
 # do that now.
 
+require 'helper'
 
-# require 'helper'
+describe 'gist' do
+  it 'has a dependency on the jist gem' do
+    Pry::Command::Gist.command_options[:requires_gem].should == "jist"
+  end
+end
 
-# describe 'gist' do
 #   before do
 #     Pad.jist_calls = {}
 #   end