update travis for Xcode 12.

[aaryankotharii]

Is your feature request related to a problem? Please describe.
Now that the project will be built using Xcode 12 and support iOS 14, we need to update travis to test in this environment.

Describe the solution you'd like

• change simulator iOS version to 14.0 ( from 13.6 )
• change Xcode version to 12.0 ( from 11.6 )

Describe alternatives you've considered
N/A

Additional context
preferably should be solved before merging #151.