Add support for commonmark 0.15.0

[onigoetz]

Also, added a full test matrix for Travis

[onigoetz]

Hi @hason,
I'm trying to make this extension compatible with commonmark 0.15.0.

With the tests I made, it seems to work fine.
But I've seen that this has a dev dependency to commonmark-twig-renderer which in trun depends to commonmark-attributes-extension which then depends on commonmark-table-extension.

This loop makes it impossible to test the chain completely. can we do something about it ?

[hason]

Thanks! I removed dependency on Twig renderer and applied your commits.