updated installation documentation for OS X 10.9 and updated date for do...

We had 2 issues installing rails due to 
```
-xcode-select --install
```
not being able to ping Apple's server and
```
gem install atomic
```
providing an error that it could not run make.

These steps outlined fixed the issues. Please let me know if you'd like me to edit the file or revert the date back to its original version (to the best of my knowledge, there is no indication of date on http://guides.railsgirls.com/install).