Update Rust requirement to 1.20.0

.travis.yml

@@ -1,6 +1,6 @@
 language: rust
 rust:
-  - 1.18.0
+  - 1.20.0
   - stable
   - beta
   - nightly

README.md

@@ -16,7 +16,7 @@ As the origin by [plietar](https://github.com/plietar/) is no longer actively ma
 More information can be found in the [wiki](https://github.com/librespot-org/librespot/wiki)
 
 # Building
-Rust 1.18.0 or later is required to build librespot.
+Rust 1.20.0 or later is required to build librespot.
 
 **If you are building librespot on macOS, the homebrew provided rust may fail due to the way in which homebrew installs rust. In this case, uninstall the homebrew version of rust and use [rustup](https://www.rustup.rs/), and librespot should then build. This should have been fixed in more recent versions of Homebrew, but we're leaving this notice here as a warning.**