Revert "travis: do build in parallel"

This reverts commit 7a1fc89.

Author: jcarpent

.travis.yml

@@ -15,7 +15,7 @@ env:
     - DO_CPPCHECK_ON_BRANCH=""
     - DO_INSTALL_DOC_EXCEPT_ON_BRANCH=""
     - BUILD_WITH_CATKIN_SUPPORT=false
-    - MAKEFLAGS="-j1"
+    - MAKEFLAGS="-j2"
 
 jobs:
   include: