Update TrueTime.podspec

[ankur-1503]

Latest XCode requires iOS 11.0 as minimum target

What did you change and why?

I am building on latest XCode, carthage fails as minimum ios target is set to 8.0

Potential risks introduced?

What tests were performed (include steps)?

Checklist

• [ ] Unit/UI tests have been written (if necessary)
• [ ] Manually tested