fix(scully-routes.service): fix missing return type

[SanderElias]

Put in the correct type for the return of the getCurrent function. In some cases TS inferrrence is
off otherwise

closes #171

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/scullyio/scully/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Other... Please describe:

What is the current behavior?

typing of the getCurrent method is off

Issue Number: #171

What is the new behavior?

It returns the correct type.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[aaronfrost]

I think you should add return types to every function and public member var of this class. This will fix countless future problems.