Bring `lead` styles more inline with GOV.UK ones

Bootstrap uses a lighter font weight for text using the lead class.
This lighter text is harder to read and looks out of place with all of
GOV.UK’s font rendering.

* Reset back to normal font weight
* Lighten text colour to maintain balance
Merge pull request #40 from alphagov/reduce-lead

Bring `lead` styles more inline with GOV.UK ones