.travis.sh: increase the number of jobs to run in parallel

The travis-ci.org vms have 2 cores so lets oversubscribe them a little
to speed things up.

Signed-off-by: Chris Packham <judge.packham@gmail.com>

.travis.sh

@@ -18,7 +18,7 @@ ct-ng_travis_build()
         .config
 
     # Build the sample
-    ct-ng build.2 &
+    ct-ng build.8 &
     local build_pid=$!
 
     # Start a runner task to print a "still running" line every 5 minutes