Add Support for Magento 2.3

[thampe]

Hi,

Magento 2.3 seems to need a higher version of the symfony/yamlpackage. So I have updated the package version constraint.

According to the composer documentation "symfony/yaml": "~3.3" should not break anything, it allows Versions from >=3.3.0 and <4.0.0.

Could you review the changes and and create a new Version, so that the extension can be installed in Magento 2.3 System?

Thanks!

[iancassidyweb]

@chevli can you take a look at this when you get a second.