Test with future parser

* Add Puppet future parser to Travis matrix (Only Puppet 3.5 for now; rspec-puppet doesn't work with anything higher, and allow failures since future parser is still pre-release.)
* Fix some bugs surfaced by the 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