Add Truffleruby head to CI

[gogainda]

No description provided.

[byroot]

Duplicate of #120 (comment)

The thing is I can't think of anything in Marginalia that would break on Truffle, that wouldn't already break in dozens if not hundreds of other gems.

If it was a stable release then maybe, but running -head without marking it as allowing failure is a support burden.

Also this makes CI 7 times slower. I really don't see the usefulness really.