Finalize zend-servicemanager forwards-compatibility changes

[weierophinney]

This builds off of #86 and #95, with the following changes:

• dependency changes:
  • allow PHP 5.5+ OR PHP 7.0+
  • Use ^2.6 versions of zend-filter, zend-validator (no need to be more specific)
• build changes:
  • run a job per servicemanager version, per PHP version (prevent regressions specific to PHP version + SM version differences)
• incorporates feedback made during review of #95
• ensures the InputFilterPluginManager constructor is backwards compatible with zend-servicemanager v2.

A few other minor refactors were made for consistency with other components and/or to clarify context.