Build: Test on more jQuery Core versions

The initial 1.14.0 announcement only promised support for the latest version
of jQuery Core within each major. The main goal was to avoid testing on too many
old versions. However, just testing on the latest one seems too drastic. For
now, add `3.6.4` to the mix; we'll work on a more detailed policy soon.

Closes gh-2258

.github/workflows/node.js.yml

@@ -26,6 +26,7 @@ jobs:
           - "git"
           - "3.x-git"
           - "3.7.1"
+          - "3.6.4"
           - "2.2.4"
           - "1.12.4"
 
@@ -68,6 +69,7 @@ jobs:
           - "git"
           - "3.x-git"
           - "3.7.1"
+          - "3.6.4"
           - "2.2.4"
           - "1.12.4"
     steps:
@@ -106,6 +108,7 @@ jobs:
           - "git"
           - "3.x-git"
           - "3.7.1"
+          - "3.6.4"
           - "2.2.4"
           - "1.12.4"
     steps: