travis.yml: Fix protobuf compilation caching

Wildcard apparently doesn't work... silently. Move versions into their
own directory so we can use a hard-coded string in the travis
configuration.
diff --git a/.travis.yml b/.travis.yml
index 04cbe92..6ee1ddd 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -55,7 +55,7 @@
 
 cache:
   directories:
-    - /tmp/protobuf-*
+    - /tmp/protobuf-cache
     - /tmp/gradle-caches-modules-2
     - /tmp/gradle-wrapper