Travis fix for pip install pep8

Fix for recent Travis image cleanup.
Issue https://github.com/travis-ci/travis-ci/issues/8378

.travis.yml

@@ -1,5 +1,4 @@
 language: cpp
-group: deprecated-2017Q3 # workaround for failing: pip install pep8
 matrix:
   include:
     - compiler: clang

Tools/travis-linux-install.sh

@@ -88,7 +88,7 @@ case "$SWIGLANG" in
 		travis_retry sudo apt-get -qq install php$VER-cli php$VER-dev
 		;;
 	"python")
-		pip install pep8
+		pip install --user pep8
 		if [[ "$PY3" ]]; then
 			travis_retry sudo apt-get install -qq python3-dev
 		fi