Tuning unicorn_custom recipe for issue-3414

We want to amend our custom unicorn recipe to allow us to tweak the
number of web workers for different environments.

In this first iteration we use the instance type (c1.medium,
c1.extra_large etc.) to decide how many web workers to configure.
Fixing up unicorn_custom recipe

- Fix typo in main recipe
- Add a check for TRANSLATION_STORE in the unicorn config
Merge pull request #1 from ExamTime/issue-3414

Issue 3414 Configuring number of web workers for different environments