Fix method name after D254

Summary:
D254 removed DifferentialReplyHandler::getRevision(), but
is still using it in two places. Correct them.

Test Plan:
send email email handler and verified it works.

Reviewed By: epriestley
Reviewers: tuomaspelkonen, epriestley
CC: aran, epriestley
Blame Revision:
D254

Differential Revision: 277