💚 Fix CI for oldest Rubies

Author: pboling

spec/snaky_hash/string_keyed_spec.rb

@@ -8,6 +8,11 @@
   it_behaves_like "a snaked hash"
 
   it "can transform keys to string" do
+    skip_for(
+      engine: "ruby",
+      versions: %w(2.2.10 2.3.8 2.4.10),
+      reason: "transform_keys is not available in these versions of Ruby"
+    )
     a = described_class.new(asd: "asd")
     b = a.transform_keys(&:to_s)
     expect(b.keys).to eq(["asd"])