Fix deprecation for symfony/config 4.2+

[helios-ag]

Q	A
Bug fix?	[no]
New feature?	[no]
BC breaks?	[no]
Deprecations?	[no]
Tests pass?	[yes]
Fixed tickets	[#346]
License	MIT

[codecov]

Codecov Report

Merging #347 into master will decrease coverage by 0.01%.
The diff coverage is 75%.

@@             Coverage Diff              @@
##             master     #347      +/-   ##
============================================
- Coverage     55.75%   55.74%   -0.02%     
- Complexity      260      261       +1     
============================================
  Files            14       14              
  Lines          1060     1062       +2     
============================================
+ Hits            591      592       +1     
- Misses          469      470       +1

Impacted Files	Coverage Δ	Complexity Δ
src/DependencyInjection/Configuration.php	97.9% <75%> (-0.23%)	20 <0> (+1)

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