Make Humbug tests up and running

[pamil]

From the lowest versions:

• PHP 5.5 build fails because of changed Symfony\Component\Console\Application::getLongVersion() from symfony/console somewhere in between 2.8.19 and 3.2.7, it should be solved by overriding its definition
• PHP 5.6 build fails because of calling unexisting PHPUnit_Util_Configuration::getTestdoxGroupConfiguration() from phpunit/phpunit somewhere in between 4.8.35 and 5.7.19, needs to be investigated

No more changes needed to get Travis green, removing hhvm-nightly and matrix configuration is already done in #207.

[pamil]

The strange failures for PHP 5.6 and PHP 7.0 are caused by Humbug using a PHPUnit binary provided with phpenv, so PHP 7.0 uses PHPUnit 6.0 even when Composer installs PHPUnit 5.7 to vendor/bin.

[padraic]

Merged both PRs for you. Should be close to having all our 3rd party service support back in order after today!

[pamil]

After having a deeper look, Composer does not return correct binaries directory because Behat changes current working directory to something like /tmp/humbug-behat6b5eee and there's no composer.json. This causes Humbug to look for phpunit using $PATH environmental variable.