Test on v1.6 (#294)

Since this is the new LTS, it might make sense to test on v1.6 specifically.

Author: jishnub

.github/workflows/UnitTest.yml

@@ -26,7 +26,7 @@ jobs:
       # https://github.com/actions/toolkit/issues/399
       fail-fast: false
       matrix:
-        julia-version: ['1.0', '1', 'nightly']
+        julia-version: ['1.0', '1.6', '1', 'nightly']
         os: [ubuntu-latest, windows-latest, macOS-latest]
         julia-arch: [x64]
         # only test one 32-bit job