Use Travis cache.

[skip ci]

.travis.yml

@@ -176,3 +176,9 @@ notifications:
     on_failure: always
     skip_join: true
 
+cache:
+  directories:
+    - installs/mono-3.2.8
+    - installs/py2
+    - installs/py3
+    - installs/perl-5.18