ci: use two parallel build jobs on Travis-CI

Change-Id: I0d37709bd81bbff890c932b77dc5be696ac35569
diff --git a/.travis.yml b/.travis.yml
index 8be7822..2feaefc 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,6 +3,7 @@
 env:
   global:
     - JOB_NAME=limited-build
+    - WAF_JOBS=2
 
 matrix:
   include: