Remove unused xpath_options parameter. Closes #588

Ruby 1.9 would silently drop the options as an extra parameter in
block.call, but Ruby 1.8 would convert the block parameters to an array,
causing breakage. The following code demonstrates the difference -- Ruby
1.9 prints 42, whereas Ruby 1.8 prints [42, {}]:

    def a(&block)
      block.call(42, {})
    end
    a do |x|
      p x
    end

lib/capybara/selector.rb

@@ -46,8 +46,8 @@ module Capybara
       @failure_message
     end
 
-    def call(locator, xpath_options={})
-      @xpath.call(locator, xpath_options)
+    def call(locator)
+      @xpath.call(locator)
     end
 
     def match?(locator)