Alternate precedence order for ZF2 path

- It makes more sense to assume that if ZF2 is actually _in_ the
  project, to use it, and only if not present start looking at the ENV
  and php.ini settings.
Merge branch '167'
Closes #167
Update ZF2 submodule to 2.1.3
Merge pull request #168 from aarbrown/zf213

Update ZF2 submodule to 2.1.3