investigate potential PATH problem on travis-ci

[sbourdeauducq]

Before I modified the code to take into account the PYTHON environment variable, the command python used in a few places ran the wrong interpreter and resulted in failing builds due to packages not found (migen, mibuild, elftools, etc.)

This may or may not be due to this:
http://irclog.whitequark.org/m-labs/2015-07-29#13556938;

Make sure that there is no funny things happening with PATH as this may lead to other bugs of this nature later.