CI: Fix accessing `json` CLI tool

Using it now depends on the working directory, it'll only work in the
repo root, but oh well.
diff --git a/circle.yml b/circle.yml
index f247988..934d387 100644
--- a/circle.yml
+++ b/circle.yml
@@ -33,7 +33,7 @@
 
 machine:
   environment:
-    json: ~/node_modules/.bin/json
+    json: node_modules/.bin/json
 
 dependencies:
   cache_directories:
@@ -60,7 +60,7 @@
         } >$CIRCLE_ARTIFACTS/sauce-connect.log 2>&1
       :
         background: true
-    - test -x $json || { cd; npm install json; }
+    - test -x $json || npm install json
 
 test:
   override: