Raise go versions in testing workflow (#349)

Author: andygrunwald

.github/workflows/testing.yml

@@ -11,7 +11,7 @@ jobs:
     name: Test and lint
     strategy:
       matrix:
-        go-version: [1.x, 1.13.x, 1.12.x]
+        go-version: [1.x, 1.15.x, 1.14.x]
         platform: [ubuntu-latest, windows-latest]
     runs-on: ${{ matrix.platform }}