Gentoo corrections and improvements

This pull request includes the following changes to the Gentoo template:

Correction tothe URL for downloading the Gentoo ISO - The format of the text file the filename is retrieved from changed.
Ruby 2.0 is not installed instead of Ruby 1.9 - This is required to install Chef.
Corrections to the URL for downloading Stage3 file - The format of the of text file for fetching the filename changed.
Updated Kernel version from 3.14.14 to 3.14.36 - The Kernel image version was also wrong for AMD64 architecture.
Uninstall Gentoo Sources as a part of the cleanup - This has shaved 120MB from the resulting box size.

jedi4ever / veewee