Remove TreeBuilder.root deprecated usage

[tomlongridge]

Goal

A tree builder without a root node is deprecated since Symfony 4.2 and will not be supported anymore in 5.0. This Pull Request fix this and allow Symfony 4.2 users to use this bundle without deprecation notices.

Changeset

Changed

• Configuration.php - now returns the root node using getRootNode if on Symfony 4.1+

Tests

• Manual regression of example projects

Review

For the submitter, initial self-review:

• [x] Commented on code changes inline explain the reasoning behind the approach
• [x] Reviewed the test cases added for completeness and possible points for discussion
• [x] A changelog entry was added for the goal of this pull request
• [ ] Check the scope of the changeset - is everything in the diff required for the pull request?
• This pull request is ready for:
  • [ ] Initial review of the intended approach, not yet feature complete
  • [ ] Structural review of the classes, functions, and properties modified
  • [x] Final review

For the pull request reviewer(s), this changeset has been reviewed for:

• [ ] Consistency across platforms for structures or concepts added or modified
• [ ] Consistency between the changeset and the goal stated above
• [ ] Internal consistency with the rest of the library - is there any overlap between existing interfaces and any which have been added?
• [ ] Usage friction - is the proposed change in usage cumbersome or complicated?
• [ ] Performance and complexity - are there any cases of unexpected O(n^3) when iterating, recursing, flat mapping, etc?
• [ ] Concurrency concerns - if components are accessed asynchronously, what issues will arise
• [ ] Thoroughness of added tests and any missing edge cases
• [ ] Idiomatic use of the language