Add Ruby 2.6 to test matrix

.travis.yml

@@ -4,7 +4,7 @@ dist: trusty
 services:
   - docker
 rvm:
-  - 2.5.3
+  - 2.6.0
   - jruby-9.2.4.0
 gemfile:
   - Gemfile

lib/capybara/spec/session/find_spec.rb

@@ -106,7 +106,7 @@ Capybara::SpecHelper.spec '#find' do
       expect(@session.find(:xpath, "//input[@id='test_field']").value).to eq('monkey')
     end
 
-    it 'should warn if passed a non-valid locator type', :focus_ do
+    it 'should warn if passed a non-valid locator type' do
       expect_any_instance_of(Kernel).to receive(:warn).with(/must respond to to_xpath or be an instance of String/)
       expect { @session.find(:xpath, 123) }.to raise_error # rubocop:disable RSpec/UnspecifiedException
     end