Build requires nightly rust

[maackle]

I could not build with a stable rust build. The README should probably mention that nightly rust is required.

Sample error:

error[E0554]: #![feature] may not be used on the stable release channel
  --> /Users/michael/.cargo/registry/src/github.com-1ecc6299db9ec823/runtime-fmt-0.3.0/src/lib.rs:17:1
   |
17 | #![feature(fmt_internals)]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^

[Connoropolous]

@zippy has tacked this on in #45