Install Python deps in Docker image.
diff --git a/tools/run_tests/tests.sh b/tools/run_tests/tests.sh
index db49975..5281cd2 100755
--- a/tools/run_tests/tests.sh
+++ b/tools/run_tests/tests.sh
@@ -152,6 +152,9 @@
 }
 
 internal_install_python_deps() {
+  if [ "$TRAVIS" != "true" ]; then
+    return;
+  fi
   # Install tox (OS X doesn't have pip).
   if [ $(uname -s) == "Darwin" ]; then
     sudo easy_install tox