Enable ccache on Travis
diff --git a/.travis.yml b/.travis.yml
index 1d1a740..1259152 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,6 +3,7 @@
 - clang
 - gcc
 sudo: false
+cache: ccache
 script:
 - tests/scripts/recursion.pl library/*.c
 - tests/scripts/check-generated-files.sh