R1:e7d2e96c23f0

Changing the method of validating sendmail exists on the system.

Summary:
Removes calling `which` and instead checks the file path that PHPMailerLite uses directly (/usr/bin/sendmail).

This fixes $PATH export issues which occur on certain platforms causing un-expected errors during setup.

Test Plan:
* Run setup on server without sendmail install & error should be presented.
* Install sendmail then re-run setup & no error should be presented.

Reviewers: epriestley

Differential Revision: 375