use pip instead of pip3 in pg-travis-test.sh

Author: funbringer

travis/pg-travis-test.sh

@@ -97,7 +97,7 @@ virtualenv /tmp/envs/pg_pathman
 source /tmp/envs/pg_pathman/bin/activate
 
 # install pip packages
-pip3 install $pip_packages
+pip install $pip_packages
 
 # run python tests
 make USE_PGXS=1 PG_CONFIG=$config_path python_tests || status=$?