Travis: make most targets run with -j2

As documented in Travis the VMs have 1.5 cores
(http://docs.travis-ci.com/user/speeding-up-the-build/#Parallelizing-your-build-on-one-VM).
So let's make it only -j2.

The exception is PX4 build that runs without any -j setting since NuttX
fails to build if we do that.

Tools/scripts/build_all_travis.sh

@@ -27,9 +27,9 @@ build_platforms=(  ["ArduPlane"]="apm2 navio sitl linux px4-v2"
                    ["AntennaTracker"]="apm2 navio sitl linux px4-v2"
                    ["Tools/Replay"]="linux")
 
-build_concurrency=(["apm2"]="-j4"
+build_concurrency=(["apm2"]="-j2"
                    ["navio"]="-j2"
-                   ["sitl"]="-j4"
+                   ["sitl"]="-j2"
                    ["linux"]="-j2"
                    ["px4-v2"]="")