Use Rust 1.51 in CI

We needed to bump the version because the `array-impls` feature 
requires const generics.

Author: LukasKalbertodt

.github/workflows/ci.yml

@@ -59,7 +59,7 @@ jobs:
       - uses: actions/checkout@v2
       - uses: sfackler/actions/rustup@master
         with:
-          version: 1.46.0
+          version: 1.51.0
       - run: echo "::set-output name=version::$(rustc --version)"
         id: rust-version
       - uses: actions/cache@v1