Bump minimum nightly to `nightly-2019-05-21`

For stabalisation of `MaybeUninit`
cf. https://github.com/rust-lang/rust/pull/60445

.travis.yml

@@ -2,7 +2,7 @@ sudo: false
 language: rust
 rust:
   - nightly
-  - nightly-2019-04-18 # Minimum supported
+  - nightly-2019-05-21 # Minimum supported
 branches:
   only:
   - master

README.md

@@ -15,6 +15,9 @@ Cross-origin resource sharing (CORS) for [Rocket](https://rocket.rs/) applicatio
 
 If you are using Rocket 0.3, use the `0.3.0` version of this crate.
 
+There is a minimum version of Rust nightly required. This is usually higher than whatever
+Rocket requires plus more if the dependent crates require other features.
+
 ### Nightly Rust
 
 Rocket requires nightly Rust. You should probably install Rust with