Cache the submodule checkout in travis
diff --git a/.travis.yml b/.travis.yml
index 49f4f04..7d24e53 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -8,6 +8,7 @@
   cargo: true
   directories:
     - tests/rust
+    - .git/modules
 
 matrix:
   allow_failures: