travis: Enable ccache

.travis.yml

@@ -1,6 +1,8 @@
 sudo: required
 language: cpp
 
+cache: ccache
+
 before_install:
   - if [ "$TRAVIS_OS_NAME" = "linux" ]; then sudo apt-get update -qq && sudo apt-get install cmake && sudo apt-get install cmake python3 && sudo apt-get install -qq freeglut3-dev libxmu-dev libxi-dev ; echo -n | openssl s_client -connect scan.coverity.com:443 | sed -ne '/-BEGIN CERTIFICATE-/,/-END CERTIFICATE-/p' | sudo tee -a /etc/ssl/certs/ca- ; fi
   - 'if [ "$TRAVIS_OS_NAME" = "osx" ];  then