Add 1.3 release date

kud1ing · kud1ing · commit da059a4e2805 · 2015-09-18T09:19:20.000+02:00
As of http://blog.rust-lang.org/2015/09/17/Rust-1.3.html
diff --git a/RELEASES.md b/RELEASES.md
@@ -1,4 +1,4 @@
-Version 1.3.0 (September 2015)
+Version 1.3.0 (2015-09-17)
 ==============================
 
 * ~900 changes, numerous bugfixes
