[growthforecast] Increase the timeout value, when `bundle exec rake test`.

Because the default timeout is set to 180 seconds,
testing timed out.

In order to workaround this problem,
set 3600 seconds (= 60 minutes) as the timeout value.

```
diff --git a/circle.yml b/circle.yml
index b15c0ee..b4be95b 100644
--- a/circle.yml
+++ b/circle.yml
@@ -17,3 +17,4 @@ test:
   override:
     - bundle exec rake foodcritic
     - bundle exec rake test
+      timeout: 3600
```