Force environment in UTF-8

Summary:
When the Jenkins server includes UTF-8 JSON characters
to API reply, for example « Dæghrefn », it broke JSON
parsing.

As Ruby is happy to use as default encoding the system
environment encoding, it's set to C.UTF-8 to fix this issue.

Reference: https://oncletom.io/2015/docker-encoding/

Test Plan:

• Add a job with an UTF-8, non ASCII character to Jenkins
• Start the new image

Reviewers: dereckson

Differential Revision: http://devcentral.nasqueron.org/D482