Update CI matrix now that sshkit has dropped Ruby < 2.5 support (#161)

Before, we were testing old versions of ruby against the master branch
of sshkit, but has now dropped official support for Ruby 2.4 and older.
Update the CI matrix to ensure we are using valid combinations of Ruby
and sshkit versions.

.github/workflows/ci.yml

@@ -20,7 +20,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby: ["2.4", "2.5", "2.6", "2.7", "3.0", "3.1", "3.2", "3.3", "3.4"]
+        ruby: ["2.5", "2.6", "2.7", "3.0", "3.1", "3.2", "3.3", "3.4"]
     env:
       sshkit: "master"
     steps:
@@ -53,8 +53,13 @@ jobs:
     runs-on: ubuntu-20.04
     strategy:
       matrix:
-        ruby: ["2.0", "2.0", "2.1", "2.2", "2.3"]
-        sshkit: ["1.6.1", "1.7.1", "master"]
+        ruby: ["2.0", "2.0", "2.1", "2.2", "2.3", "2.4"]
+        sshkit: ["1.6.1", "1.7.1", "1.22.0"]
+        exclude:
+          - ruby: "2.4"
+            sshkit: "1.6.1"
+          - ruby: "2.4"
+            sshkit: "1.7.1"
     env:
       sshkit: ${{ matrix.sshkit }}
     steps: