Support beta Rust release. Closes #8 (#11)

* Remove seemingly unneeded const on MetadataKey<T> new

* Change biome temperature/moisture to integer, x100 to remove floating-point so can use within stable 'const fn'

* Remove unstable const_fn feature, now using stable const fn: see https://www.reddit.com/r/rust/comments/9msqfn/const_fn_soon_on_stable_rust/

* Test on Rust beta (awaiting 1.31 release for stable)

* Update readme for beta Rust support

protocol/src/types/metadata.rs

@@ -29,7 +29,7 @@ pub struct MetadataKey<T: MetaValue> {
 
 impl <T: MetaValue> MetadataKey<T> {
     #[allow(dead_code)]
-    const fn new(index: i32) -> MetadataKey<T> {
+    fn new(index: i32) -> MetadataKey<T> {
         MetadataKey {
             index: index,
             ty: PhantomData,