Added dependacy for mysql-chef-gem

It seems that you need ruby191-dev, or mkmf, installed otherwise
this will bomb out.

The error this resolves is this:
```ruby
        /usr/bin/ruby1.9.1 extconf.rb
/usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require': cannot load such file -- mkmf (LoadError)
	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from extconf.rb:5:in `<main>'
```