fixed could not get correct img file name bugs

* Test on trusty/osx on Travis

* Fix trusty installation of pyqt4

* Fix test on macOS 10.11

* Fix path of python executable

* Use system python

* Set PYTHON and PIP

* Install numpy before installing scikit-image

* Install Cython for scikit-image

* Install fortran for pip install numpy

* Install via PIP

* Use pyenv

* Fix installation of pyqt

* Install scikit-image

* Set backend of matplotlib