Use `dist` property from swagger-ui package.

This removes the hardcoded local `node_modules` lookup for swagger-ui,
which will break if the parent installs swagger-ui itself.

This is useful if, for example, the latest swagger-ui breaks
loopback-explorer, and the parent project wants to install a fixed
version in its root.
Merge pull request #37 from STRML/swagger-ui-dist

Use `dist` property from swagger-ui package.