Support no_std

[koushiro]

What have I changed?

1. Migrate to 2018 edition
2. Support no_std (rustc 1.36 has stabilize alloc crate)
3. Modify CI config

[KokaKiwi]

It still needs alloc and i would like to reach a point where alloc could be an optional dependency too, but it can be done later i suppose.
But the changes seems OK so i'll merge this, thanks 💚

PS: Also thanks for modifying Travis-CI config file, i don't use it much so i don't really know its capabilities

[fouge]

+1