Added find_xpath to make compatible with edge capybara 2.1

Currently click_link throws the following with edge capybara:

```
     Failure/Error: current_email.click_link "Unsubscribe"
     NotImplementedError:
       NotImplementedError
     # /Users/karl/.rvm/gems/ruby-1.9.3-p327-falcon@viewthespace/bundler/gems/capybara-d73c24d3e3fb/lib/capybara/driver/base.rb:11:in `find_xpath'
     # /Users/karl/.rvm/gems/ruby-1.9.3-p327-falcon@viewthespace/bundler/gems/capybara-d73c24d3e3fb/lib/capybara/node/finders.rb:154:in `block in resolve_query'
     # /Users/karl/.rvm/gems/ruby-1.9.3-p327-falcon@viewthespace/bundler/gems/capybara-d73c24d3e3fb/lib/capybara/node/base.rb:78:in `synchronize'
```