Ensure CommandT::MRU is defined

Even if the extension isn't compiled, we want/need CommandT::MRU to be
defined otherwise our autocommands will blow up as reported here:

  https://github.com/wincent/Command-T/issues/92

I don't particularly like how subtle/brittle this is (the implicit order
dependency), but for now this minimal fix will have to do.

There is still something else going on with issue #92; waiting on
further information about there.