Unignore double click tests on Firefox

They now pass on Travis using GeckoDriver 0.20.0 and Firefox 59.0

Author: p0deje

rb/spec/integration/selenium/webdriver/action_builder_spec.rb

@@ -141,8 +141,7 @@ module WebDriver
           expect(text).to eq('Dropped!')
         end
 
-        # Pending bug with Firefox
-        it 'double clicks an element', except: {browser: :firefox} do
+        it 'double clicks an element' do
           driver.navigate.to url_for('javascriptPage.html')
           element = driver.find_element(id: 'doubleClickField')