Fix GitHub CI build

* Use stable branch of Rust instead of broken minimum required nightly
  version.
* Disable fail-fast to reveal all problems at once.
* Remove deletion of rust-toolchain file as the file is no longer
  required/existing.

.github/workflows/rust.yml

@@ -11,8 +11,8 @@ jobs:
     strategy:
       matrix:
         rust:
+          - stable
           - nightly
-          - nightly-2019-05-21  # MSRV
         os:
           - ubuntu-latest
           - windows-latest
@@ -22,6 +22,8 @@ jobs:
           - "--all-features"
           - "--no-default-features"
 
+      fail-fast: false
+
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v1
@@ -35,9 +37,6 @@ jobs:
           override: true
           components: rustfmt, clippy
 
-      - name: Remove Rust Toolchain file
-        run: rm rust-toolchain
-
       - uses: actions-rs/cargo@v1
         name: Clippy Lint
         with: