Use pep8 in travis builds

Check the following errors/warnings (see [docs](http://pep8.readthedocs.org/)):
* E101 - mix of tabs and spaces
* W191 - use of tabs
* W291 - trailing whitespace
* W293 - trailing whitespace
* E111 - 4 spaces per indentation level
* E112 - 4 spaces per indentation level
* E113 - 4 spaces per indentation level
* W292 - no newline at end of file
* W391 - blank line at end of file

N.B.: this set of errors was adopted from the astopy project.