alert: add alert-dismissible class

[mattscilipoti]

Per https://getbootstrap.com/components/#alerts-dismissible, dismissible alert boxes should have an alert-dismissable css class.

[coveralls]

Coverage remained the same at 100.0% when pulling 150b8c7 on mattscilipoti:alert_dismissible into 88a6398 on Fullscreen:master.

[claudiofullscreen]

@mattscilipoti Is this what you are looking for?

http://fullscreen.github.io/bh/#dismissible-alerts

[mattscilipoti]

@claudiofullscreen Yes. That element is just missing the 'alert-dismissible' class, that bootstrap specifies should be there. My pull request simply adds that class. Bootstrap has some styling for that class.

[mattscilipoti]

The broken travis build is having an issue installing json. I don't think this small change is related to that issue.

[mattscilipoti]

Here is the listed failure from travis-ci. It is not related to the changes in this PR. I expect it is affecting other PRs too. I don't know how you are managing multiple rubies, but it looks like something isn't configured correctly.:

...
Gem::InstallError: json requires Ruby version ~> 2.0.
Installing tilt 1.4.1
An error occurred while installing json (2.0.2), and Bundler cannot continue.
Make sure that `gem install json -v '2.0.2'` succeeds before bundling.
The command "eval bundle install --jobs=3 --retry=3" failed. Retrying, 2 of 3.
...

Please review.