Add -Ilib to default rcov_opts

Created by: tianyicui

I'm always happy to have this line in my Rakefile:

RSpec::Core::RakeTask.new('rspec')

But when I want to add a rcov task like this:

RSpec::Core::RakeTask.new('rcov') do |t| t.rcov = true end

It won't work. Because 'lib' is not in $: - rcov won't search there until you tell it to.

IMO, if a Rakefile contains the rspec task as showed above, the rcov task that only tweaked the rcov property should be work out of box. So I suggest to add -Ilib to the default options for the runner rcov, because the rspec runner will search there by default.

Thanks.