Update release schedule for 2016.

Mozilla just updated theirs, and I assume we'll go on aligning ours on Tor
Browser's that will itself be aligned to Mozilla's:

https://blog.mozilla.org/futurereleases/2016/02/04/update-on-2016-firefox-release-schedule/

wiki/src/contribute/calendar.mdwn

@@ -4,8 +4,14 @@
 
 * 2016-04-19: Release 2.3
 
-* 2016-05-31: Release 2.4
+* 2016-06-07: Release 2.4
 
-* 2016-07-12: Release 2.5
+* 2016-08-02: Release 2.5
 
-* 2016-08-23: Release 2.6
+* 2016-09-13: Release 2.6
+
+* 2016-11-08: Release 2.7
+
+* 2016-12-13: Release 2.8
+
+* 2017-01-24: Release 2.9