Provide PHP 7.1 support

[weierophinney]

This patch accomplishes the following:

• Updates dependencies
  • Drops support for PHP versions prior to 7.1.
  • Updates to zend-expressive-template v2 series.
  • Updates to zend-expressive-router v3 series.
  • Updates to zend-expressive-helpers v5 series.
  • Updates to PHPUnit 6.5 series.
• Updates to use PHP 7.1 features:
  • Adds strict_types declarations to all class files
  • Updates to use scalar and return type hints, including nullable and void types, wherever possible. In some cases, where extending classes from components that have not updated to PHP 7.1 syntax yet, typehints were omitted for compatibility.
  • Ensures ZendViewRenderer implements v2 signature of zend-expressive-template TemplateRendererInterface.