Update minimal Rust version from 1.31 to 1.51

Author: kpp

.github/workflows/build.yml

@@ -105,7 +105,7 @@ jobs:
         toolchain:
           - stable
           - nightly
-          - 1.36.0
+          - 1.51.0
         features:
           - serde
         buildtype:
@@ -242,7 +242,7 @@ jobs:
       - name: Install Rust
         uses: actions-rs/toolchain@v1
         with:
-          toolchain: 1.36.0
+          toolchain: 1.51.0
           target: x86_64-unknown-linux-gnu
           override: true