config: fix cache path

The path to cache was missing a slash, meaning it was looking for a
"dot folder" instead of a relative path which ultimately led to
an empty cache.

Author: jbergstroem

.circleci/config.yml

@@ -18,7 +18,7 @@ jobs:
       - save_cache:
           key: dependency-cache-{{ checksum "package.json" }}
           paths:
-            - .node_modules
+            - ./node_modules
       - run:
           name: test
           command: npm test