Replace URI.encode with ERB::Util.url_encode

URI.encode is an alias for URI.escape which has been deprecated. There
is a great stackoverflow question discussing this deprecation and
possible replacements, http://stackoverflow.com/a/2832003/1769125. 

After some testing I decided that ERB::Util.url_encode was the best choice as
it most closely mirrored the current implementation.

```ruby
foo = "http://google.com?query=hello"

old_way = URI.escape(foo, Regexp.new("[^#{URI::PATTERN::UNRESERVED}]"))

new_way = ERB::Util.url_encode(foo)

old_way == new_way
```