Use singular 'repository' key to avoid warning

Hi, it seems that the key `repositories` in package.json is not supported and gives a warning during `npm install`.

    npm WARN package.json remotestorage-oauth@0.1.4 'repositories' (plural) Not supported.
    npm WARN package.json Please pick one as the 'repository' field

This updates to use the singular `repository`. Reference: https://github.com/isaacs/npm/issues/2624