[build] drop 1.9.3; Travis: use dist: trusty

[olleolleolle]

This PR does the smallest change to the Travis configuration to get a build on the new default environment.

• use dist: trusty
• remove 1.9.3 support (using commonmarker does that)

The result is a build with no "errors" only build "failures".

[olleolleolle]

• 1.9.3 Gem::InstallError: commonmarker requires Ruby version >= 2.0.0.
• 2.x, ruby-head: Test failure, things have changed

[gravis]

Thanks for you help!