Update podspec to 6.1, migrate to XCTest, fix project config for new Coc...

Update podspec to 6.1, migrate to XCTest, fix project config for new Cocoapods version.

This is basically a cleanup change: The travis build started failing and I believe it was due to some Cocoapods changes. The config changes are to fix the new Cocoapods requirements, and while testing that I found that the unit test was out of date, so fixed that, and then had to convert it to XCTest since SenTestKit is apparently deprecated.

@hariharsubramanyam any chance you have time to take a look at this? (no rush!)