Switch to luarocks installation method for Debian and derivatives

Debian's LuaSocket packages are broken.  Then seem to
package the 5.1 version of the library as simply a copy
of the 5.2 version; however, the libraries are incompatible.
Additionally, the package seems to take a number of
different names depending on the version of the distro.
However, the LuaRocks package seems to be fine, so install
that first and then install LuaSocket through LuaRocks.

README.md

@@ -30,7 +30,9 @@ many distributions, for example:
 	# # On Arch Linux:
 	# pacman -S lua51-socket
 	# # On Debian/Ubuntu:
-	# apt-get install lua-socket
+	# # Debian/Ubuntu's LuaSocket packages are broken, so use LuaRocks.
+	# apt-get install luarocks
+	# luarocks install luasocket
 
 You will also need to add IRC to your trusted mods if you haven't disabled mod
 security.  Here's an example configuration line: