ci: preserve PATH when running waf under sudo

Change-Id: Id178dba19d34b8f5f76de34cfe8407078e2f3409
Refs: #4186
diff --git a/.travis.yml b/.travis.yml
index d5ff924..8be7822 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -135,6 +135,7 @@
 
 before_script:
   - ${CXX:-c++} --version
+  - python --version
 
 script:
   - ./.jenkins