Bugfix npm warn about "repositories" field

npm sends these warnings about the *repositories* field :

    npm WARN package.json sleep@1.1.2 'repositories' (plural) Not supported.
    npm WARN package.json Please pick one as the 'repository' field

As I don't know if the *repositories* field is needed for other purposes, I prefer adding a new **repository** field.

Feel free to delete the plural form if not needed.