New .travis.yml to use travis-perl helpers.

[bigpresh]

I think this will fail initially, but let's see.

The branch build failed: https://travis-ci.org/PerlDancer/Dancer/builds/357342445

I'm not sure how to see why - the build log shows e.g.:

cpanm --quiet --installdeps --notest .
! Configuring . failed. See /home/travis/.cpanm/work/1521803992.2438/build.log for details.

... but I assume there's no way for me to see the output that was generated in cpanm's build log. Hmm.