Add support for mail 2.6.1

mail 2.6.0+ replaces treetop parser with a Ragel based parser.

Instead of checking domain dot_atom_text contains more than 1 element, assert domain contains more than 1 elements split by dot.

Also fixed rspec to 2.99 to avoid breaking changes in rspec 3.
From what i understand, the comment addresses the fact that a domain should be made up of at least two dot atom text elements, which are atext followed by an optional period "." , and the way it was done was to dig into the treetop parse tree and check that.

Now that mail moved away from treetop, splitting the domain by period and checking that there are two or more elements achieves that same purpose.