Update version of source-map, remove @types reference

[dylans]

New versions of source-map include their own type definitions, so we can remove our dependency on type definitions if we update to a new version of source-map:

npm WARN deprecated @types/source-map@0.5.7: This is a stub types definition for source-map (https://github.com/mozilla/source-map). source-map provides its own type definitions, so you don't need @types/source-map installed!

[matt-gadd]

I think this is just a superfluous @type's dependency as far as I can tell, so this can just be removed.