use checkout@v2 in github workflow (#21)

Author: Roguelazer

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - name: Checkout
-      uses: actions/checkout@v1
+      uses: actions/checkout@v2
     - name: Install rust
       uses: actions-rs/toolchain@v1
       with: