Update actions/checkout requirement to v2.3.1 (#20)

* Update actions/checkout requirement to v2.3.1

Updates the requirements on [actions/checkout](https://github.com/actions/checkout) to permit the latest version.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/master/CHANGELOG.md)
- [Commits](28c7f3d2b5)

Signed-off-by: dependabot[bot] <support@github.com>

* Fix the version numbers

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Deirdre Connolly <deirdre@zfnd.org>

.github/workflows/main.yml

@@ -18,7 +18,7 @@ jobs:
         os: [ubuntu-latest, windows-latest, macOS-latest]
 
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v2.3.1
       - uses: actions-rs/toolchain@v1
         with:
           toolchain: stable
@@ -42,7 +42,7 @@ jobs:
     name: Run doctests via nightly
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@master
+    - uses: actions/checkout@v2.3.1
     # Because we use nightly features for building docs,
     # using --all-features will fail without nightly toolchain.
     - uses: actions-rs/toolchain@v1
@@ -58,7 +58,7 @@ jobs:
     name: Code Coverage
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v2.3.1
       - uses: actions-rs/toolchain@v1
         with:
           toolchain: stable
@@ -77,7 +77,7 @@ jobs:
     name: Clippy
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v2.3.1
       - uses: actions-rs/toolchain@v1
         with:
           toolchain: stable