Fixes syntax error for 1.8.7

The only use of Aruba::Config::Hooks#execute was in `api.rb` and
that was passing a single value for the second argument.

So, I removed the splat in the arg and the call to
`instance_exec`.  Since none of this is documented, and no tests
broke, I'm going to assume this is OK, at least to get
a working version for 1.8.7.

lib/aruba/config.rb

@@ -24,9 +24,9 @@ def append(label, block)
       @store[label] << block
     end
 
-    def execute(label, *args, context)
+    def execute(label, command, context)
       @store[label].each do |block|
-        context.instance_exec(*args, &block)
+        context.instance_exec(command, &block)
       end
     end
   end
@@ -41,4 +41,4 @@ def configure
   end
 
   self.config = Config.new
-end
+end