travis: move some builds to the first thread

Now that we are build every linux board twice (one with make and one
with waf) and additionally building all examples and tests, it's taking
a long time to leave all linux boards in the same thread. Let's move
some of them to the other thread.

.travis.yml

@@ -89,5 +89,5 @@ env:
     #   via the "travis encrypt" command using the project repo's public key
     - secure: "FjIwqZQV2FhNPWYITX5LZXTE38yYqBaQdbm3QmbEg/30wnPTm1ZOLIU7o/aSvX615ImR8kHoryvFPDQDWc6wWfqTEs3Ytq2kIvcIJS2Y5l/0PFfpWJoH5gRd6hDThnoi+1oVMLvj1+bhn4yFlCCQ2vT/jxoGfiQqqgvHtv4fLzI="
   matrix:
-    - TRAVIS_BUILD_TARGET="px4-v2"
-    - TRAVIS_BUILD_TARGET="sitl linux navio raspilot minlure bebop"
+    - TRAVIS_BUILD_TARGET="px4-v2 sitl linux"
+    - TRAVIS_BUILD_TARGET="navio raspilot minlure bebop"