chore(travis): make travis cache ccache and use it for all compiling

osx builds fail with ccache, so they don't use it.

.travis.yml

@@ -1,6 +1,7 @@
 sudo: required
 dist: trusty
 language: cpp
+cache: ccache
 
 # regex is for release tags
 branches:

.travis/build-ubuntu-14-04.sh

@@ -48,6 +48,10 @@ sudo apt-get install -y --force-yes \
     qt53xmlpatterns \
     pkg-config || yes
 
+# make sure to use ccache for all the compiling
+export CC="ccache $CC"
+export CXX="ccache $CXX"
+
 # Qt
 source /opt/qt53/bin/qt53-env.sh || yes