Enable pep8 check with travis

I don't like your approach: it doubles the already big number of test case. as pep8 check should be quite fast, I advocate for removing travis_test.sh (I don't like having too many trabvis-related stuff in the source, I already dislike .travis.yml in fact) and put the following in the script section:
```
script:
    - printf "[user]\ncxx=$CXX\ncxxflags=$CXXFLAGS\n" > ~/.pythranrc
    - PYTHONPATH=$PWD:$PYTHONPATH py.test pythran --pep8 -m pep8
    - OMP_NUM_THREADS=4 PYTHONPATH=$PWD:$PYTHONPATH py.test -n 2 --pep8 -v $TESTCASE
```