Require time lib so util method can convert to Time objects (#38)

In the `Util.deep_parse_iso8601_values` we reference `Time.iso8601` but
to use that in Ruby you need to require the time library otherwise the
method is undefined. The implementation has a `rescue` but that was
unintended since its only meant to capture cases where a string can’t
be converted to a time object. We weren’t seeing any errors here
because it also caught the undefined error and let the time be a string
value.

Author: javierjulio

lib/dwolla_v2.rb

@@ -3,6 +3,7 @@
 require "json"
 require "forwardable"
 require "ostruct"
+require "time"
 
 require "hashie"