Dependency update

[hafkenscheid]

Updated dependencies, fixed dependency related things, added an exception and added phpdocs.

[hafkenscheid]

For issue #114

[rvanlaak]

Thank you for the draft PR. I see it does a lot of different things, from providing phpdoc, to adding infra dependencies to composer.json, and dropping earlier versions of dependencies and their quirks. Imho we should split this to different PRs, to get simple changes merged quickly.

About the phpdoc changes; all the simple typehinted object types will be deducted from the PHP typehints by most IDEs. So, the phpdocs that were added there are not needed when no additional description gets provided. Would you be ok with reverting those changes?

[hafkenscheid]

I reverted the PHPdoc changes.

Do you also want to split the dependencies and related updates?
In my opinion it is not very useful to check full symfony 3.4 compatibility because it is not being maintained anymore.

[rvanlaak]

v3.0.0-beta3 does some household cleanup as well, and introduces Github Actions for testing as Travis stopped running tests. https://github.com/dmishh/SettingsBundle/releases/tag/v3.0.0-beta3

Regarding 3.4 compatibility, does the 3.4 support block you from introducing new features? As long as it takes no effort to keep support, I'm in favor of keeping 3.4 under test coverage.

Could you please revise this PR so we can get it merged?