ServiceManager v2-v3 compatibility

- to ensure no regressions between PHP versions as well.

- ensure compat with both PHP 5.5+ and 7.0+
- Use `^2.6` (no maintenance version), as we don't need to get more specific
  at this time.
- remove empty line at end of file

- Created a ternary from the original if/else, making the logic more compact.
- Added the method docblock.

- Move v2 checks into v2-specific methods of abstract factory.
- Use provided `$services` argument when lazy-instantiating filter and
  validator plugin managers.
- For abstract factory tests, vary methods tested based on service
  manager version.

- Renamed `MigrationTest` to `InputFilterPluginManagerCompatibilityTest`
  (consistency with other components, and this is specifically for
  testing migration of the plugin manager).
- Added tests to validate that the `InputFilterPluginManager` accepts no
  arguments and/or a `ConfigInterface` argument to the constructor when
  using zend-servicemanager v2.
- Updated the `InputFilterPluginManager` constructor to mirror that of
  the 2.7 and 3.0 series of zend-servicemanager; it also populates the
  initializers prior to calling the parent constructor, allowing
  overriding.

From what I can tell, it was added for purposes of testing; I updated
the test to use `assertAttributeSame`, varying the property to test
against based on SM version.