Switch underscore-mixin for lodash-mixin

This package isn't being tracked within https://github.com/AmpersandJS/ampersand/issues/36 but it is dependant upon the [ampersand-collection-underscore-mixin](https://github.com/AmpersandJS/ampersand-collection-underscore-mixin). I've switched this in favour of the [ampersand-collection-lodash-mixin](https://github.com/STRML/ampersand-collection-lodash-mixin).

Currently the lodash mixin is pulling in all of lodash, I'm not sure if this is the right approach.