ci: preserve PATH when running waf under sudo

Change-Id: I090dd49d1d8f78007b1b7684af7563fbedac770a
Refs: #4186
diff --git a/.travis.yml b/.travis.yml
index abd85fe..aeea85b 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -135,6 +135,7 @@
 
 before_script:
   - ${CXX:-c++} --version
+  - python --version
 
 script:
   - ./.jenkins