Building in PHP 7.2

[Sam-Burns]

• Fixes broken 5.6 build by running project's PHPUnit version rather than Travis' version
• Bumps PHPUnit version to one that works in PHP 7.2
• Drops HipHop build
• Adds builds for PHP 7.1 & 7.2

Fix #88

[michalbundyra]

It is already done on develop, but unfortunately it is not merged yet. See: https://github.com/zendframework/zend-stdlib/compare/develop

[weierophinney]

#87 does this and more, but is on develop as it removes support for HHVM.

Sorry I missed that during triage, @Sam-Burns !