Update ChangeLog for 4.0.4.

Author: jasone

ChangeLog

@@ -4,7 +4,12 @@ brevity.  Much more detail can be found in the git revision history:
 
     https://github.com/jemalloc/jemalloc
 
-* 4.0.4 (XXX)
+* 4.0.4 (October 24, 2015)
+
+  This bugfix release fixes another xallocx() regression.  No other regressions
+  have come to light in over a month, so this is likely a good starting point
+  for people who prefer to wait for "dot one" releases with all the major issues
+  shaken out.
 
   Bug fixes:
   - Fix xallocx(..., MALLOCX_ZERO to zero the last full trailing page of large