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.
I don't understand this comment and so cannot tell whether this commit changes the behaviour or not and if it does how.

If behaviour has changed it would be good to add a comment into the commit message to explain how and why this is not important.