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