travis: On Python3 use python3-version of pep8 #38

Previously pep8 and pyflakes were installed using the default system Python.
This changes 'make travis' to use the current Python version for those scripts as well.
This affects pep8, at least: on Python3 it complains about space in `print (...)`.
Replaced print with print_function from __future__, refs #38.
socket.py: pep8 fix on python3 #38
test__all__.py: pep8 fix on python3 #38
util/pyflakes.py: do not ignore errors, py3&py2.6

- do not swallow syntax errors
- make pyflakes.py py3-compatible #38
- run using specific version of python (previously pyflakes from the default python
  was used, which caused it to ignore py3-specific and py2.6-specific syntax errors
fix syntax of xtest_signal.py on python2.6