[WIP] Bump minimum version to PHP 7.1

[asgrim]

container-interop-doctrine doesn't seem to be installable on my PHP 7.1 system for some reason:

$ composer require dasprid/container-interop-doctrine
Using version ^1.0 for dasprid/container-interop-doctrine
./composer.json has been updated
Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - doctrine/instantiator 1.0.1 requires php ~5.3 -> your PHP version (7.1.7) does not satisfy that requirement.
    - doctrine/instantiator 1.0.3 requires php ~5.3 -> your PHP version (7.1.7) does not satisfy that requirement.
    - doctrine/instantiator 1.0.2 requires php ~5.3 -> your PHP version (7.1.7) does not satisfy that requirement.
    - Conclusion: don't install doctrine/orm v2.5.0
    - Installation request for dasprid/container-interop-doctrine ^1.0 -> satisfiable by dasprid/container-interop-doctrine[1.0.0].
    - Conclusion: don't install doctrine/instantiator 1.1.0|install doctrine/orm v2.5.0
    - Conclusion: remove doctrine/instantiator 1.1.0|install doctrine/orm v2.5.0
    - dasprid/container-interop-doctrine 1.0.0 requires doctrine/orm ^2.5 -> satisfiable by doctrine/orm[v2.5.0, v2.5.1, v2.5.2, v2.5.3, v2.5.4, v2.5.5, v2.5.6].
    - doctrine/orm v2.5.6 requires doctrine/instantiator ~1.0.1 -> satisfiable by doctrine/instantiator[1.0.1, 1.0.2, 1.0.3, 1.0.4, 1.0.5].
    - doctrine/orm v2.5.5 requires doctrine/instantiator ~1.0.1 -> satisfiable by doctrine/instantiator[1.0.1, 1.0.2, 1.0.3, 1.0.4, 1.0.5].
    - doctrine/orm v2.5.4 requires doctrine/instantiator ~1.0.1 -> satisfiable by doctrine/instantiator[1.0.1, 1.0.2, 1.0.3, 1.0.4, 1.0.5].
    - doctrine/orm v2.5.3 requires doctrine/instantiator ~1.0.1 -> satisfiable by doctrine/instantiator[1.0.1, 1.0.2, 1.0.3, 1.0.4, 1.0.5].
    - doctrine/orm v2.5.2 requires doctrine/instantiator ~1.0.1 -> satisfiable by doctrine/instantiator[1.0.1, 1.0.2, 1.0.3, 1.0.4, 1.0.5].
    - doctrine/orm v2.5.1 requires doctrine/instantiator ~1.0.1 -> satisfiable by doctrine/instantiator[1.0.1, 1.0.2, 1.0.3, 1.0.4, 1.0.5].
    - Can only install one of: doctrine/instantiator[1.0.4, 1.1.0].
    - Can only install one of: doctrine/instantiator[1.0.5, 1.1.0].
    - Installation request for doctrine/instantiator (locked at 1.1.0) -> satisfiable by doctrine/instantiator[1.1.0].

Installation failed, reverting ./composer.json to its original content.

Therefore this PR bumps the minimum PHP version to 7.1.

However, at this moment in time, we still need a newer ORM release to be able to merge this.

[coveralls]

Coverage decreased (-0.9%) to 45.631% when pulling 69c5521e15a1c37a0894127d41a53c347a0de461 on asgrim:php-7.1 into a4833d25dcfa42165c4ddec0c7062528206eeadc on DASPRiD:master.

[asgrim]

https://gist.github.com/asgrim/e3ad8ff1d71ce5e3e7685a3e1443ab27

[asgrim]

Gonna close this; not sure this is really a problem :)