Add Named Users Uninstall API Client

[mtkasima]

What does this do and why?

Added because Named User uninstall API Client was not in gem

Testing

• [x] I wrote tests covering these changes

• I've tested for Ruby versions:

• [x] 2.2.5

• [x] 2.3.1

Urban Airship Contribution Agreement

https://docs.urbanairship.com/contribution-agreement/

• [x] I've filled out and signed UA's contribution agreement form.

[sarahdactyl71]

All tests pass. LGTM