Raise minimum rustc version in CI to 1.59

Required by simd-json's dependency on value-trait:

    error: package `value-trait v0.2.12` cannot be built because it
    requires rustc 1.59 or newer, while the currently active rustc
    version is 1.56.0

Author: dtolnay

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        rust: [nightly, beta, stable, 1.56.0]
+        rust: [nightly, beta, stable, 1.59.0]
     steps:
       - uses: actions/checkout@v2
       - uses: dtolnay/rust-toolchain@master