Use pyqt=5.6 on macOS travis to fix unknown error

```
INTERNALERROR> Traceback (most recent call last):
INTERNALERROR>   File "/Users/travis/miniconda/envs/test/lib/python3.6/site-packages/_pytest/main.py", line 181, in wrap_session
INTERNALERROR>     config._do_configure()
INTERNALERROR>   File "/Users/travis/miniconda/envs/test/lib/python3.6/site-packages/_pytest/config/__init__.py", line 629, in _do_configure
INTERNALERROR>     self.hook.pytest_configure.call_historic(kwargs=dict(config=self))
INTERNALERROR>   File "/Users/travis/miniconda/envs/test/lib/python3.6/site-packages/pluggy/hooks.py", line 306, in call_historic
INTERNALERROR>     res = self._hookexec(self, self.get_hookimpls(), kwargs)
INTERNALERROR>   File "/Users/travis/miniconda/envs/test/lib/python3.6/site-packages/pluggy/manager.py", line 67, in _hookexec
INTERNALERROR>     return self._inner_hookexec(hook, methods, kwargs)
INTERNALERROR>   File "/Users/travis/miniconda/envs/test/lib/python3.6/site-packages/pluggy/manager.py", line 61, in <lambda>
INTERNALERROR>     firstresult=hook.spec.opts.get("firstresult") if hook.spec else False,
INTERNALERROR>   File "/Users/travis/miniconda/envs/test/lib/python3.6/site-packages/pluggy/callers.py", line 208, in _multicall
INTERNALERROR>     return outcome.get_result()
INTERNALERROR>   File "/Users/travis/miniconda/envs/test/lib/python3.6/site-packages/pluggy/callers.py", line 80, in get_result
INTERNALERROR>     raise ex[1].with_traceback(ex[2])
INTERNALERROR>   File "/Users/travis/miniconda/envs/test/lib/python3.6/site-packages/pluggy/callers.py", line 187, in _multicall
INTERNALERROR>     res = hook_impl.function(*args)
INTERNALERROR>   File "/Users/travis/miniconda/envs/test/lib/python3.6/site-packages/pytestqt/plugin.py", line 218, in pytest_configure
INTERNALERROR>     qt_api.set_qt_api(config.getini("qt_api"))
INTERNALERROR>   File "/Users/travis/miniconda/envs/test/lib/python3.6/site-packages/pytestqt/qt_compat.py", line 111, in set_qt_api
INTERNALERROR>     self.QtCore = QtCore = _import_module("QtCore")
INTERNALERROR>   File "/Users/travis/miniconda/envs/test/lib/python3.6/site-packages/pytestqt/qt_compat.py", line 95, in _import_module
INTERNALERROR>     m = __import__(_root_module, globals(), locals(), [module_name], 0)
INTERNALERROR> ImportError: dlopen(/Users/travis/miniconda/envs/test/lib/python3.6/site-packages/PyQt5/QtCore.so, 2): Library not loaded: @rpath/libQt5Core.5.dylib
INTERNALERROR>   Referenced from: /Users/travis/miniconda/envs/test/lib/python3.6/site-packages/PyQt5/QtCore.so
INTERNALERROR>   Reason: image not found
```

.travis.yml

@@ -26,7 +26,7 @@ matrix:
     - os: osx
       env:
         - PYTEST_QT_API=pyqt5
-        - PYQT_PACKAGE='pyqt=5'
+        - PYQT_PACKAGE='pyqt=5.6'
         - PYTHON_VERSION=3.6
         - RUN_PYINSTALLER=true
     - os: linux