TravisCI: remove xcode7.3 on DPL mac build

.travis.yml

@@ -269,7 +269,6 @@ matrix:
 # Deployer Buildbots - OSX
 ################################
         - os: osx
-          osx_image: xcode7.3
           if: env(DPL_ENABLED) = "1" AND (env(_DPL_JOB_ENABLED) = "1" OR env(DPL_JOB_ENABLE_ALL) = "1")
               AND (branch =~ /^.*deployer.*$/ OR (tag IS present AND env(DPL_TAG_ENABLED) = "1"))
               AND env(DPL_TERMINATE_MAIN) != "1"