ci: use two parallel build jobs on Travis-CI

Change-Id: I734c5826f37e1027484ef9723933e8954dba00a8
diff --git a/.travis.yml b/.travis.yml
index aeea85b..0dfe8ba 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,6 +3,7 @@
 env:
   global:
     - JOB_NAME=limited-build
+    - WAF_JOBS=2
 
 matrix:
   include: