Update Changelog for 1.3.0

Author: bkeepers

HISTORY.md

@@ -1,6 +1,10 @@
-## 1.3.0 (unreleased)
+## 1.3.0 (2014-09-11)
 
+* Extend the field limit for tables to 50 characters for RST [#306](https://github.com/github/markup/pull/306)
+* Add `.mkdn` as a supported markdown extension [#308](https://github.com/github/markup/pull/308)
 * Upgrade wikicloth to 0.8.1 [#317](https://github.com/github/markup/pull/317)
+* Force encoding of posix-spawn output [#338](https://github.com/github/markup/pull/338)
+* Add `.rmd` as a supported markdown extension [#343](https://github.com/github/markup/pull/343)
 
 [Full changelog](https://github.com/github/markup/compare/v1.2.1...v1.3.0)