add Puppet future parser to Travis matrix

Only Puppet 3.5 for now; rspec-puppet doesn't work with anything higher.
Allow failures since future parser is still pre-release.
bug fixes surfaced by future parser

* concat::fragment requires 'order' to be a string
* init parameters added in #373 referred to wrong variable name in params.pp
* cannot perform regex match on a number, so make {fastcgi,proxy}_cache_levels
  a string by default
* bug fixed in test suite
Merge pull request #438 from 3flex/future-parser

Test with future parser