Include the overlapping xpath for ClickeFailed from obscured elements

This makes it easier to debug failed clicks because of modals, etc.

Example message:

    Failed to click element /html/body/div[@id='one'] because of overlapping
    element /html/body/div[@id='two'] at position 199, 199
Only require 'webrick/cookie' in order to avoid long load times on ruby 2.0.0-p195
Merge pull request #545 from pivotal-cloudplanner/master

Only require 'webrick/cookie' in order to avoid long load times on ruby 2.0.0-p195
Improve failure message when trying to click invisible elements

When not ignoring invisible elements, the message didn't explain that
the click failed because the element was invisible.
Bump selenium-webdriver

Closes #500