Issue title: Plan information fails to load on non-USD/EUR accounts with Premium upgrade
 Issue body: #### Steps to reproduce
1. Use an account configured for a non-USD/EUR currency with a Premium plan (other plans seem fine)
2. Open URL: https://wordpress.com/plans/my-plan/

#### What I expected
To see the plan features for my selected plan

#### What happened instead
The plan information never loads.  After 2 minutes the API call to https://public-api.wordpress.com/rest/v1.1/sites/`SITEID`/plans?http_envelope=1&locale=en returns with a 504 response

It seems like the problem started after this commit: https://github.com/Automattic/wp-calypso/commit/0e7744c66e5d836e9885086324e12f8dda366246

#### Browser / OS version
OSX Chrome

#### Screenshot / Video
![screen shot 2016-08-25 at 9 53 00 am](https://cloud.githubusercontent.com/assets/7233112/17973086/0ee8e152-6aaf-11e6-9bc3-2f8378143c50.png)
![screen shot 2016-08-25 at 9 56 16 am](https://cloud.githubusercontent.com/assets/7233112/17973090/111acde6-6aaf-11e6-8999-13be951b0f32.png)

#### Context / Source
#automated-testing
 Comments: 
 Comment 0: This was tracked to a backend issue with changeset r141262-wpcom.

Moved this over to Trac ticket 7778-wpcom
{'source': 'GithubIssues', 'repo_name_with_owner': 'Automattic/wp-calypso', 'issue_id': 173220541, 'repo_stars': 13604.0}