Turn on -Werror on Travis and Appveyor

[ndmitchell]

So that such warnings don't sneak out.

[vincenthz]

only on the main compiler (now 8.0) though, . there's lots of warnings on old versions (now < 8.0) that are not very relevant, and next version (now 8.2) has the tendency to introduce new warnings which are somewhat not too useful / too noisy

[ThreeFx]

I'll take a look at this. (On a side note: Funnily enough, I get warnings for orphan instances...) Nevermind, I was looking at the wrong options.