gnuplot: accept lua or versions/lua52 as dependency

Gnuplot depends on Lua (we currently install 5.1.5), but Lua 5.2 from homebrew-versions seems to work fine as well (I tried [this](http://zeekish.wordpress.com/2011/01/22/quick-notes-on-using-gnuplots-lua-terminal/)). This PR uses a Requirement to make an either-or dependency.