- Updates dependencies to versions known to work with 2.7 when possible.- Fixes a signature within the PluginManager- Fixes a test asset signature- Adds 7.1 and 7.2 jobs- Drops PHP 5.5 from the matrix  Most dependencies _require_ 5.6 as a minimum supported version, and thus  we cannot install dependencies as a result.  It may still _work_ on 5.5, but if you're on 5.5, you don't care about  7.2 support.- Use latest possible version of PHPUnit in the v4 series.

…n detected`PluginManager` overrides the `get()` method, which has a differentsignature in v2 than in v3. Until PHP 7.2, this was not a problem;however, in 7.2, even optional values must follow the exact samesignature as the parent. As a result, we now need to vary theimplementations.This patch provides `AbstractPluginManager`, which contains the bulk of thelogic for implementing the `PluginManager`. It then introduces thefollowing:- `PluginManagerSM2`, which extends `AbstractPluginManager` and  implements the zend-servicemanager v2 `get()` signature.- `PluginManagerSM3`, which extends `AbstractPluginManager` and  implements the zend-servicemanager v3 `get()` signature.It removes the `PluginManager` class.It then adds a file-based autoloader that checks to see if`Zend\ServiceManager\PluginManagerInterface` exists. If it does, italiases `PluginManager` to `PluginManagerSM3`; otherwise, it aliases itto `PluginManagerSM2`.Additionally, this patch updates to zend-servicemanager 2.7.10, whichhas important fixes to how it handles creation options withInvokableFactory that are necessary here.