Multiplatform testing

I did the same thing [as with luassert](https://github.com/Olivine-Labs/luassert/pull/53) but ran into some problems. https://travis-ci.org/mpeterv/busted/builds/19993063

First, on Lua 5.1 and LuaJIT 2.0 [one test](https://github.com/Olivine-Labs/busted/blob/master/spec/generic_async_test.lua#L273-282) in the async test fails. It was failing before, too, maybe it should be removed? 

Another problem is that there is a bug(or misdocumentation) in penlight which causes some failures with Lua 5.2. [I sent a fix](https://github.com/stevedonovan/Penlight/pull/105) but it'll take time to get it into a release, so I'll try to adapt the tests to this issue.