Improve assets for compatibility with symfony 3.4/4.0

[gregurco]

Assets are not installed automatically in symfony 3.4/4.0.

• [x] add symfony/framework-bundle in requirements (there is defined assets:install command and symfony/asset package in dependencies)
• [x] add assets:install command in recipe (source: https://github.com/symfony/recipes#composer-scripts-configurator)

[florianpreusner]

Part 1 released in v7.3.0 (https://github.com/8p/EightPointsGuzzleBundle/releases/tag/7.3.0)

[florianpreusner]

Part 2 is not necessary cause assets:install will be run by framework-bundle. https://github.com/symfony/recipes-contrib/pull/512#discussion_r221609209

Thanks for the hint, @gregurco! 🙌