www/seamonkey: update to 2.40

SeaMonkey now uses FFmpeg directly rather than via GStreamer for H.264
and MP3 playback as part of HTML5. Note, FFmpeg 3.x is unsupported[1].

Changes: http://www.seamonkey-project.org/news#2016-03-14
PR: 210931
Submitted by: net@arrishq.net (based on)
Security: 2c2d1c39-1396-459a-91f5-ca03ee7c64c6
MFH: 2016Q3

[1] SeaMonkey 2.40 is based on Firefox 43 codebase but Firefox 45

is the earliest to have the fix. Backporting maybe tricky:
https://bugzilla.mozilla.org/show_bug.cgi?id=1232268