Run tests on only a single Rust version, since we are relying on compiler messages

Author: andrewhickman

.github/workflows/ci.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        rust_version: [stable, "1.39.0"]
+        rust_version: [stable]
     steps:
     - uses: actions/checkout@v1
     - name: Setup Rust toolchain
@@ -24,4 +24,3 @@ jobs:
       run: |
         cargo fmt -- --check
         cargo clippy
-      if: matrix.rust_version == 'stable'