Travis CI: skip Pylint under Python 3.6

With astroid<=1.4.9, it crashes when encountering `import traceback`.
With astroid master, it produces a whole lot of spurious errors
and then still crashes.

Let's wait until astroid==1.5.0 is released.

HACKING.rst

@@ -15,7 +15,7 @@ Run tests::
 
   $ pytest
 
-Run Pylint::
+Run Pylint (currently doesn't work under Python 3.6)::
 
   $ tools/pylint_all.sh -j 2
   $ # ... or selectively:

tools/pylint_all.sh

@@ -4,4 +4,11 @@ set -e
 
 test -e setup.py || { echo 'must run from repo root' >&2; exit 1; }
 
+if [ "$TRAVIS_PYTHON_VERSION" = 3.6 ]; then
+    # Pylint on HTTPolice doesn't work under Python 3.6
+    # with the current versions of astroid (1.4.9 and master).
+    echo 'WARNING: skipping Pylint under Python 3.6' >&2
+    exit 0
+fi
+
 find . -name '*.py' | grep -Fv ./doc/conf.py | xargs pylint "$@"