Updates BASE_URL

[gmaffia]

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What does this do and why?

Updates the BASE_URL and SERVER hostnames from go.airship.com to go.urbanairship.com based on the support ticket #117

Testing

• [x] If these changes added new functionality, I tested them against the live API with real auth

• [ ] I wrote tests covering these changes

• I've tested for Ruby versions:

• [ ] 2.2.5

• [x] 2.3.1

Airship Contribution Agreement

Link here

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

Screenshots

• If applicable

[sarahdactyl71]

LGTM

[aschuman0]

Looks like I was beaten to the review, but: Thank you very much for the PR @gmaffia! We'll get this out asap.