Warn when trying to compile with a non-nightly rustc

[zekesonxx]

I am so [expletive deleted] and tired of spending twenty minutes trying to figure out what the [expletive] is wrong before eventually realizing I didn't rustup override set nightly.

So having a clear warning when you're not using xargo with a nightly compiler would be helpful.

[japaric]

master already has a warning for this (#168) but there won't be a new stable release until #167 is fixed.