Modifying Travis-CI configuration to use  build stages and removing
explicit MySQL 5.6 execution (it's default now).

PHPCS is running with PHP nightly version so we can have it failing
for now.

This is now mandatory on the new Travis-CI containers.

Removes all sudo items (except hhvm which currently still requires sudo)
reverting the other simpler matrix attempt

- Suspend HHVM testing until the following items are resolved
  - HHVM PHP 7 mode issues.
    - facebook/hhvm#7626 php 7 mode not recognized by composer as PHP 7
    - facebook/hhvm#7198 type annotations on internal functions, `expects parameter 1 to be _______, null given`
    - facebook/hhvm#7544 HHVM INI settings for PHP7 mode `php 7 is not the same as php 7.1`
  - Trusty Container issues (currently beta)
    - https://github.com/travis-ci/travis-ci/issues/6842 - MySQL socket issue on Trusty beta Container (assume fixed sometime after 1Q17)
- Add line to Force hhvm PHP 7 mode for future testing
  - HHVM fails on composer `This package requires php ^7.0 but your HHVM version does not satisfy that requirement`
  - reference for change https://docs.hhvm.com/hhvm/configuration/INI-settings#php-7-settings

php 7.1 with MARIADB_VERSION=10.1 seems to no longer be erroring randomly on tests

to correctly install and test the pgsql versions it looks like we are gong to have to long form the matrix with each of the pgsql versions

try to only add when pgsql

POSTGRESQL_VERSION=9.6 was released September 2016 See if we can run it on travis

Additionally allowing failures on PHP 7.1 with mariadb MARIADB_VERSION=10.1 since it is intermittently erroring

- fixes version of HHVM being tested
- drops mariadb 5.5
- adds POSTGRESQL_VERSION=9.5 (only available with sudo enabled trusty)