Use renamed pylintrc file that is only meant for Travis checks.
Check all Python scripts in 'addons', 'htmlreport' and 'tools'
Errors for `_socketobject` class are disabled, see
https://stackoverflow.com/questions/10300082/how-to-prevent-python-pylint-complaining-about-socket-class-sendall-method
Install imported modules `unittest2` and `pexpect` via pip.
Add "./addons" to search-path for modules because
"tools/compare-ast-clang-and-cppcheck.py" imports cppcheckdata.py from
addons. Pylint does not seem to evaluate
`sys.path.insert(0, '../addons')` in the script. So an `init-hook` is
necessary in pylintrc_travis.
Sebastian