Drop support for PHP version < 7.1

[okonomi]

For next major version, I want to drop supporting PHP 7.0 or lower.

• Support for PHP 5.5 or lower is already stopped
• Security support for PHP 5.6, 7.0 will also end this year
• Major libraries and frameworks are support for PHP 7.1 or higher
  • https://packagist.org/packages/laravel/laravel
    • php: ^7.1.3
  • https://packagist.org/packages/symfony/symfony
    • php: ^7.1.3
  • https://packagist.org/packages/phpunit/phpunit
    • php: ^7.1
• If it is limited to 7.1 or higher support, it will be possible to use some features.
  • ex) short array syntax, return type declarations, etc...

[coveralls]

Coverage decreased (-1.06%) to 97.527% when pulling 5014787 on php71 into 8b7d9f5 on master.

[mimmi20]

ex) short array syntax, return type declarations, etc.

Does hhvm support these features?

[okonomi]

@mimmi20 good point! I was missing considering hhvm compatibility...

"short array syntax" and "return type declarations" are ordinary syntax in today's PHP.
In hhvm official website, hhvm seems to support the major features of PHP 7, so probably we can use that.

But I do not know much about hhvm. I am going to experiment once about this subject.