Added swift_version to PromisesSwift.podspec. Fix of #150.

[edudnyk]

Fixing issue #150 where cocoapods require swift_version to be specified if Podfile declares :inherit_targets => false.

The issue #150 is the result of the issue with target settings validation in CocoaPods that is decided not to be fixed.

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[ykjchen]

@edudnyk thanks for proposing the change, could you also sign the CLA?

[edudnyk]

@edudnyk thanks for proposing the change, could you also sign the CLA?

Already did.

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[edudnyk]

@googlebot I signed it!

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We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google. In order to pass this check, please resolve this problem and then comment @googlebot I fixed it.. If the bot doesn't comment, it means it doesn't think anything has changed.

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[ykjchen]

Thanks!