Set update of minor versions as default option

[mtodor]

Fixes #5.

[chrfritsch]

I used drupal 8.4.x to test this. When i am using the --major-versions option, i got this output afterwards

composer outdated -D
doctrine/annotations v1.2.7 v1.4.0 Docblock Annotations Parser
doctrine/common v2.5.3 v2.7.2 Common Library for Doctrine projects
egulias/email-validator 1.2.14 2.1.2 A library for validating emails
symfony-cmf/routing 1.4.0 2.0.0 Extends the Symfony2 routing component for dynamic routes and chaining several rou...
symfony/class-loader v2.8.18 v3.2.6 Symfony ClassLoader Component
symfony/console v2.8.18 v3.2.6 Symfony Console Component
symfony/dependency-injection v2.8.18 v3.2.6 Symfony DependencyInjection Component
symfony/dom-crawler v2.8.18 v3.2.6 Symfony DomCrawler Component
symfony/event-dispatcher v2.8.18 v3.2.6 Symfony EventDispatcher Component
symfony/http-foundation v2.8.18 v3.2.6 Symfony HttpFoundation Component
symfony/http-kernel v2.8.18 v3.2.6 Symfony HttpKernel Component
symfony/process v2.8.18 v3.2.6 Symfony Process Component
symfony/psr-http-message-bridge v0.2 v1.0.0 PSR HTTP message bridge
symfony/routing v2.8.18 v3.2.6 Symfony Routing Component
symfony/serializer v2.8.18 v3.2.6 Symfony Serializer Component
symfony/translation v2.8.18 v3.2.6 Symfony Translation Component
symfony/validator v2.8.18 v3.2.6 Symfony Validator Component
symfony/yaml v2.8.18 v3.2.6 Symfony Yaml Component
twig/twig v1.33.0 v2.3.0 Twig, the flexible, fast, and secure template language for PHP

[chrfritsch]

Ok, i tested this with our thunder-project template and it works fine.