Use the skype installer with version for old MacOS version

I found the `getskype-macosx.dmg` installer cannot work on Mountain Lion.
When I tried to start Skype, the dialog like below opened:
![latest_skype_needs_mavericks_or_later](https://cloud.githubusercontent.com/assets/19299/4196064/1399d214-37cc-11e4-8616-8ccc5afcea15.png)
It says "This application needs OS X 10.9 or later".
So I modified this formula to use `getskype-macosx.dmg` for MacOS.version >= "10.9" and `Skype_6.15.0.335.dmg` otherwise.

Could you review this?
Thanks!