Upgrade from Ubuntu 14.04 (Trusty) to Ubuntu 16.04 (Xenial) (#601)

### Upgrade from Ubuntu 14.04 (Trusty) to Ubuntu 16.04 (Xenial)

* **NOTE**: These changes require `Xenial` to be used from this point forward. `Trusty` will no longer work as there are _various incompatibilities_.

* The platform is now exclusively supported on Ubuntu 16.04 (Xenial).  Trusty (14.04) is no longer supported after this commit.

* HHVM has been upgraded to to version 3.21.

* Provision, Docker, and Vagrant configurations have been updated to support 16.04. This includes Travis builds which utilizes the Docker image (PR #569).

* Provision Changes:

  * HHVM repo and repo keys updated for Xenial

  * HHVM "restarted" instead of "started" after HHVM repo mode enabled. Enabling repo mode appears to start HHVM before permissions are applied to repo file. Therefore it must be restarted.

  * `osquery` repo source updated for Xenial

  * `node.js` legacy purge removed before installation, as this is not required for Xenial

  * Package `language-pack-en` removed, this is installed and no longer required.

  * Package `php5-cli` replaced with `php7.0-cli` for compatibility with Xenial.

  * MySQL configuration file changed from `/etc/mysql/my.cnf` to `/etc/mysql/mysql.conf.d/mysqld.cnf` for compatibility with Xenial.

* Vagrant Changes:

  * Base VM changed to Xenial

* Docker Changes:

  * Images for standalone and multi-setup changed to Xenial

  * Web directory permissions set to `www-data`. Otherwise, the web directory will be owned by root and cause permissions issues.

  * MySQL permissions enforced on `/var/lib/mysql and /var/run/mysql`. Otherwise, these directories will be owned by root and MySQL will not function.

  * HHVM explicitly started as `www-data`. Docker does not allow HHVM startup (running under root) to spawn an HHVM process under another user (www-data).

  * `sudo` and `apt-utils` installed. These are required packages for provision.

  * Based on recommended HHVM DockerFile, `ENV HHVM_DISABLE_NUMA` parameter added and set to true.