[.travis.yml] install GDAL Python binding inside virtual environment
[.travis.yml] install GDAL Python binding inside virtual environment

* [.travis.yml] dist: bionic, python: 3.7, no site-packages

* [.travis.yml] apt install swig instead of swig2.0

* [.travis.yml] apt install python3-dev, fixed bug in Python-GDAL install

* [.travis.yml] shortened the list of apt-installed packages

fixes #25 

johntruckenbrodt/pyroSAR,johntruckenbrodt/pyroSAR