Update CHANGELOG

CHANGELOG.md

 ## Version History
 
+* Abort early for invalid TMPDIR
+* Enable compiling Ruby 1.8 on OS X 10.8+ without extra flags
+* Detect number of CPU cores used for `make`
+* Fix installing Ruby 2.1.0 from trunk
+* Install Rake and Bundler in isolation when required
+* Clearer error message when HTTP download fails
+* Set default MAKE=gmake on FreeBSD
+* Support relative path as install prefix
+* Use libyaml from Homebrew if available
+
 #### 20131024
 * Added a definition for JRuby 1.7.6
 * Added a definition for Rubinius 2.1.0