gem-config: capybara-webkit: qt48 -> qt59

> WARNING: The next major version of capybara-webkit will require at
> least version 5.0 of Qt. You're using version 4.8.7.

I went to 5.9 instead of 5.11 because 5.11 doesn't currently build on
Darwin, whereas 5.9 can build on both Darwin and Linux, and is still
well within the >=5.0 requirement.