Update selenium-webdriver

updated selenium-webdriver gem, since without it the tests failed(reproduced on multiple machines):

> (::) failed steps (::)
>
> unable to obtain stable firefox connection in 60 seconds (127.0.0.1:7055) (Selenium::WebDriver::Error::WebDriverError)
> features/show_statistics.feature:10:in `Given I have logged in as "testuser" with password "secret"'
> 
> unable to obtain stable firefox connection in 60 seconds (127.0.0.1:7055) (Selenium::WebDriver::Error::WebDriverError)
> features/show_statistics.feature:10:in `Given I have logged in as "testuser" with password "secret"'
> 
> Failing Scenarios:
> cucumber features/show_statistics.feature:63 # Scenario: I can edit the todos selected from a chart
> cucumber features/show_statistics.feature:74 # Scenario: Marking a todo selected from a chart as complete will remove it from the page
> 
> 7 scenarios (2 failed, 5 passed)
> 69 steps (2 failed, 15 skipped, 52 passed)