specs: fix ambiguous argument warning

Author: ktheory

spec/ipcat_iprange_spec.rb

@@ -20,7 +20,7 @@
     it("should match first")   { (range <=> range.first).must_equal 0 }
     it("should match last")    { (range <=> range.last).must_equal 0 }
     it("should match first-1") { (range <=> range.first - 1).must_equal 1 }
-    it("should match last+1")  { (range <=> range.last + 1).must_equal -1 }
+    it("should match last+1")  { (range <=> range.last + 1).must_equal(-1) }
   end
 end