* Relax numpy version requirements

* trying to make apt-get happy

* Remove requirements.txt installation

This installs numpy, which now comes from travis.yml

* Move numpy's dependency to appveyer

* Install numpy on OSX

* Drop numpy required version to 1.11.1 for Pythons that aren't 3.6

* Make sure that the test depends match the build depends

There was a small mistake in build-wheels.sh. It looped trough available
python's and built opencv for each of them. The loop is not needed since
multibuild sets the correct python to the PATH for the Docker container.