Install Ruby 2.0 using Ubuntu PPA packages

Compared to compiling Ruby from source with ruby-build, this change speeds up provisioning of the Vagrant box by a factor of 3 to 4.

	vagrant@practicingruby:~$ ruby --version
	ruby 2.0.0p353 (2013-11-22 revision 43784) [x86_64-linux-gnu]
	vagrant@practicingruby:~$ gem --version
	2.0.14
	vagrant@practicingruby:~$ ruby-switch --check
	Currently using: ruby2.0
	------------------------

	ruby2.0-> /usr/bin/ruby2.0
	gem-> /usr/bin/gem2.0