www/firefox: update to 49.0 (rc2)

Bump PORTREVISION to account for slightly different behavior in other
gecko@ ports and to get a discrete regression window.

Changes: https://www.mozilla.org/firefox/49.0/releasenotes/
PR: 211792
Submitted by: Jochen Neumeister <joneum@bsdproject.de> (based on)
Security: 2c57c47e-8bb3-4694-83c8-9fc3abad3964
MFH: 2016Q3