Update CI to Julia 1.6 and 1.7

Author: mewilhel

.github/workflows/ci.yml

@@ -14,14 +14,19 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-
         include:
           - version: '1.6'
             os: ubuntu-latest
             arch: x64
           - version: '1.6'
             os: windows-latest
             arch: x64
+          - version: '1.7'
+            os: ubuntu-latest
+            arch: x64
+          - version: '1.7'
+            os: windows-latest
+            arch: x64
     steps:
       - uses: actions/checkout@v2
       - uses: julia-actions/setup-julia@v1