Drop support for Nim < 1.6

Because the stint library has done as well,
and because it's not versioned we can't
remain on an older version of stint.

.github/workflows/ci.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        nim: [stable, 1.4.8, 1.2.14]
+        nim: [1.6.12, stable]
     steps:
     - uses: actions/checkout@v2
     - uses: iffy/install-nim@v3