travis: pin pylint to version 1.7.4 and astroid 1.5.2
travis: pin pylint to version 1.7.4 and astroid 1.5.2

Pylint 1.8.1 has a bug https://github.com/PyCQA/pylint/issues/1773
that breaks with bare exception re-raises. Pin to a specific working
version until the fixed pylint is available on PyPI.

ohmu/pghoard,ohmu/pghoard,saaros/pghoard