Explicitly declare marketplaceService dependencies.

Defense against mangling by eventual JS minimization.

For context see:
 * https://docs.angularjs.org/tutorial/step_05 (A Note on Minification).
 * https://docs.angularjs.org/guide/di

This is an improvement I inadvertently implemented in coding #34 , and while not needed for that and indeed not needed at all until we get to a minifying build, wanted to capture the good practice change.

There's a `$inject` alternative to this array syntax for proofing dependency injection against minification, but I read somewhere that this array syntax is the preferred approach.  Having trouble finding that reference as I write this pull request description; if it's important can doubtless dredge it up again.