[3.6] Travis: use ccache (GH-3307) (#3332)

(cherry picked from commit 8adc73c)

diff --git a/.travis.yml b/.travis.yml
index 2515d6416a7f25f47763d84ea5e82664de43ec93..4a507c995e3333b4fa42088d64a1e29ba1238a6a 100644 (file)
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,8 +3,10 @@ dist: trusty
 sudo: false
 group: beta
 
-# To cache doc-building dependencies.
-cache: pip
+# To cache doc-building dependencies and C compiler output.
+cache:
+    - pip
+    - ccache
 
 branches:
   only: