WIP: #99 - remove php 5.6, 7.0 and 7.1 support

[dennybrandes]

• remove php 5.6 and 7.0
• change .travis config

[weierophinney]

This is a BC break, and the title does not reflect what it is doing:

• Bumping the minimum supported version of PHP to 7.2
• Incorrectly removing all Travis environments except 7.3 (it MUST keep at least 7.2 if it will still target 7.2!)
• Adopting PHP 7.1+ features, including strict_types, return type hints and scalar type hints

Regarding the latter, what is done is incomplete, and only addresses a small subset of the classes in this library.

The only way I'll accept this is if it does a thorough update to PHP 7.2 across all classes. Doing it piecemeal across multiple patches will inevitably leave us with classes and interfaces that do not have all type hints reflected correctly, and/or where we haven't done work to ensure type hints can be used (e.g., modifying AbstractOptions and/or ParameterObjectInterface to be iterable, so that setFromArray() can become setFromIterable(iterable $options) : AbstractOptions). Any API changes also MUST be documented in a migration document.