Require rust 1.27 or later
The libm crate, as used by nalgebra's dependency on alga, doesn't work on rust 1.26. This is what is causing build failures on openSUSE Tumbleweed. CC @alatiera.
I think we need to bite the bullet and require Rust 1.27.
openSUSE does need to update its Rust compiler; this will hopefully happen soon.