Fixes HTML formatter

[prsimp]

First off, thanks for all your hard work, Rubocop-team!

It seems the assets directory was inadvertently removed from the list of files to include with the gem in e23cac6. Without the assets directory, the --format html option is broken:

No such file or directory @ rb_sysopen - ~/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/rubocop-0.37.1/assets/output.html.erb
~/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/rubocop-0.37.1/lib/rubocop/formatter/html_formatter.rb:55:in `read'
~/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/rubocop-0.37.1/lib/rubocop/formatter/html_formatter.rb:55:in `render_html'
~/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/rubocop-0.37.1/lib/rubocop/formatter/html_formatter.rb:49:in `finished'
~/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/rubocop-0.37.1/lib/rubocop/formatter/formatter_set.rb:30:in `block (3 levels) in <class:FormatterSet>'
~/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/rubocop-0.37.1/lib/rubocop/formatter/formatter_set.rb:30:in `each'
~/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/rubocop-0.37.1/lib/rubocop/formatter/formatter_set.rb:30:in `block (2 levels) in <class:FormatterSet>'
~/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/rubocop-0.37.1/lib/rubocop/runner.rb:68:in `inspect_files'
~/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/rubocop-0.37.1/lib/rubocop/runner.rb:35:in `run'
~/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/rubocop-0.37.1/lib/rubocop/cli.rb:30:in `run'
~/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/rubocop-0.37.1/bin/rubocop:14:in `block in <top (required)>'
~/.rbenv/versions/2.3.0/lib/ruby/2.3.0/benchmark.rb:308:in `realtime'
~/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/rubocop-0.37.1/bin/rubocop:13:in `<top (required)>'
~/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/bin/rubocop:23:in `load'
~/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/bin/rubocop:23:in `<main>'

I haven't tested exhaustively to ensure that no other required files are missing, I just happened to notice this as it broke one of my scripts.

This change does introduce one new Rubocop issue (line length) but I wasn't sure what sort of formatting you'd prefer, so I've left it as-is for now:

rubocop.gemspec:20:81: C: Line is too long. [81/80]
  s.files = `git ls-files assets bin config lib LICENSE.txt README.md`.split($RS)

[bbatsov]

This line became too long (therefore the failing build).

[bbatsov]

Please add a changelog entry and update the commit message to reflect our commit message guidelines (Adds -> Add).

[bbatsov]

You'll need to rebase your PR on top of the current master. I see your changelog file is out-of-date and you added the entry in the wrong place.

[prsimp]

@bbatsov -- thanks for the quick feedback. I think it should be good-to-go now.