update travis (.travis.yml)

[jorgejesus]

Description

.travis.yml needs update to supporte flake8 and/or only packages like we have in pygeoapi
https://github.com/geopython/pygeoapi/blob/master/.travis.yml

Environment

travis testing env

• operating system:
• Python version:
  2.7, 3.6
• PyWPS version:
• source/distribution
• [x ] git clone
• Debian
• PyPI
• zip/tar.gz
• other (please specify):
• web server
• Apache/mod_wsgi
• CGI
• [x ] other (please specify): travis

Steps to Reproduce

Push to github

Additional Information