Use Pronto to check code conventions

This way developers can run the checks on their machines and using
`bundle exec` we guarantee the right versions of all our gems are being
used; with Hound we can only use the versions supported by their
service.

When including the pronto-erb_lint gem, we're getting errors in
development where our ERB does not follow the conventions Better HTML
expects. Since we only use Better HTML because ERB Lint depends on it,
and right now we are not ready to follow its conventions, we're
disabling it.

Note pronto depends on rugged, which requires CMake and pkg-config to
build the `libgit2` library it depends on. CMake and pkg-config are
installed by default in some GNU/Linux distributions like Ubuntu, but
might not be installed on other systems, so we're adding them as
development dependencies.

CONTRIBUTING.md

@@ -34,7 +34,7 @@ If you'd like us to review your pull request in good spirits, please follow our
 
 * Include specs to test any changes you've made
 * Our CI will check whether the rest of the application is still working properly; check its build and make sure all tests are passing
-* Your pull request will be automatically reviewed by Hound CI; fix any issues it reports
+* Run `bundle exec pronto run` and fix any issues it reports; these issues will also be automatically reported on the pull request
 * Follow [the seven rules of a great commit message](https://chris.beams.io/posts/git-commit/)
 
 When we review your pull request and ask for changes, if you're proficient using `git rebase` edit existing commits instead of adding new ones. If you aren't proficient with `git rebase`, ignore this point.