Relax Doctrine version constraints

[gapple]

Set constraint to minimum minor version, so that PHP 5.6+ environments can use newer minor versions.

Travis testing against a PHP 5.5.9 environment should ensure that the constraints remain valid for the minimum supported version.

Fixes #3365

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You can test locally by setting the platform config parameter in the composer.json for a project that requires drupal/console.

Updating from PHP 5.5 to PHP 5.6:

• doctrine/collections (v1.3.0 => v1.4.0)
• doctrine/common (v2.6.2 => v2.7.3)
• zendframework/zend-stdlib (3.0.1 => 3.1.0)
• zendframework/zend-feed (2.7.0 => 2.8.0)

Updating from PHP 5.6 to PHP 7.1:

• doctrine/inflector (v1.1.0 => v1.2.0)
• doctrine/collections (v1.4.0 => v1.5.0)
• doctrine/cache (v1.6.2 => v1.7.0)
• doctrine/common (v2.7.3 => v2.8.0)
• doctrine/instantiator (1.0.5 => 1.1.0)