fix circleci cache directory list

environment variable expansion doesn't work in the cache_directories section.

License: MIT
Signed-off-by: Steven Allen <steven@stebalien.com>

circle.yml

@@ -31,7 +31,7 @@ dependencies:
 
   cache_directories:
     - ~/go1.9.2.linux-amd64.tar.gz
-    - "$HOME/.go_workspace/src/gx/ipfs"
+    - ~/.go_workspace/src/gx/ipfs
 
 test:
   override: