Replace hardcoded `localhost` in spec helper.

This test had a hardcoded values for the URI to visit which varies for different test cases. When these specs are integrated in custom driver's test suite like capybara-mechanize, it leads to failures.

This replaces #1227, which was rejected for changing the original purpose of the test. However, this patch only changes the hardcoding. It will allow https://github.com/jeroenvandijk/capybara-mechanize/issues/55 to proceed and bump it's compatibility to beyond capybara 2.1.
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