Replace config_path() by generic code

[DaazKu]

config_path() does not exists in "Vanilla" lumen.
It may be defined by some plugins but by default it does not exists.

Fixes:

[shalvah]

I assume 'basePath' does not return a path with a trailing slash?

[codecov-io]

Codecov Report

Merging #398 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #398   +/-   ##
=========================================
  Coverage     91.32%   91.32%           
  Complexity      204      204           
=========================================
  Files            10       10           
  Lines           519      519           
=========================================
  Hits            474      474           
  Misses           45       45

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