Tighten up the gem dependencies for the immediate release.
Replace usage of a constant with spec support.
Fix order dependence: reset Capybara class variable to expected value, not nil.
Check Capybara default setting for bubbling up exceptions.

In the capybara-mechanize testsuite, we turn this off to pass the capybara test suite. However, this maintains backward compatibility since the default value of `Capybara.raise_server_errors` is true.