Explicitly quote "From" name part when submitting mail to the Mailgun API

Summary:
We are submitting epriestley (Evan Priestley) <noreply@meta.phacility.com>, but should be submitting "epriestley (Evan Priestley)" <noreply@meta.phacility.com>.

Add the missing quotes.

Test Plan: Locally, this makes the API calls work against the Mailgun sandbox domain.

Reviewers: chad, amckinley

Reviewed By: chad, amckinley

Differential Revision: https://secure.phabricator.com/D17831