[2.7] Travis: use ccache (GH-3307) (#3333)

(cherry picked from commit 8adc73c)

diff --git a/.travis.yml b/.travis.yml
index d73cb9e318945e04b07f46507218507384517ee7..571dd045e2de1d725e41a06e262ea3181ab4f29c 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: