Remove warning about unused local variable.

This removes the warning that was occurring by passing the `HTMLFormatter#binding` to a erb template without using that variable in the method.

The binding given to the erb template is that of a basic object that has no methods or variables apart from accessor methods. This hides the internals of`HTMLFormatter` from the ERB template.

This change was tested locally against the cucumber-ruby-core project.

Please consider a 0.8.1 release as this warning has propagated down to other projects that depend on simplecov-html.