Disable optimistic caching for meteor --get-ready on Circle CI.

We (@abernix and I) suspect the intermittent "died unexpectedly" failures
of `meteor --get-ready` on Circle CI are due to hitting their (low) limit
on the maximum number of open files.

Although optimistic caching speeds up rebuilds considerably, it doesn't do
much for initial builds, and it definitely keeps more files open.
Disabling it here seems worth a try.

circle.yml

@@ -12,7 +12,7 @@ dependencies:
     - ".babel-cache"
   override:
     # shouldn't take longer than 10 minutes
-    - METEOR_PRETTY_OUTPUT=0 ./meteor --get-ready:
+    - METEOR_PRETTY_OUTPUT=0 METEOR_DISABLE_OPTIMISTIC_CACHING=1 ./meteor --get-ready:
         timeout: 600
 
 test: