Upgrade sprockets due to security vulnerability

Created by: pmather

What does this Pull Request do?

Upgrade the current version of the sprockets Gem due to CVE-2018-3760

What's the changes?

The version of the sprockets Gem in Gemfile.lock is upgraded from version 3.7.1 to 3.7.2.

How should this be tested?

Deploy the compel application in production mode and check that it is running correctly.

Interested parties

@whunter