chore: add 2.7 to travis and bump node/ubuntu versions
chore: add 2.7 to travis and bump node/ubuntu versions

Updating distro and node to use LTS versions.

Adding 2.7 ruby to test matrix.

Dropping 2.3 from test matrix (as it has difficulties in testing). There
shouldn't be any issues on the 2.3 side though.

kevinjalbert/pronto-stylelint