Add JS integration test for "pledging opens a new Amazon tab"

[leesharma]

Context

When you click the pledge button, you should see something similar to this:

That Amazon tab opening is one of the most critical features of this app, but right now, it's untested.

Issue Details

This issue is to add a feature test that verifies that clicking that button opens a new tab. This will require enabling javascript for the test and probably installing some js testing dependency (selenium, capybara-webkit, etc.). If an external dependency is needed, you'll also need to update the README and possibly bin/setup with instructions.

Ideally, this solution will be cross-platform and/or auto-skip the test for folks without the required dependencies. It definitely needs to run properly on Travis CI!

[emilianoLeite]

@leesharma I'd be happy to take a stab at this