Update spec of MoPubSDK with 1.12.1.0

old pull request: https://github.com/CocoaPods/Specs/pull/2289

Hi, I added MoPubSDK 1.12.1.0 spec.
I made some improvements compared with the above old pull request.
- Old pull request had some missed files. That is fixed
- Old pull request had an old MoPubSDK version (1.12.0.1). Now is 1.12.1.0

Also I've checked by ```rake lint```
and I integrated MoPubSDK successfully by using this spec.

Best regards,