Refactored smells out of query_runner/selection.rb

Smells reported by reek:

Before:

 1: NoBrainer::QueryRunner::Selection has no descriptive comment
    (IrresponsibleModule)
 3: NoBrainer::QueryRunner::Selection#call calls env[:query] 3 times
    (DuplicateMethodCall)
 2: NoBrainer::QueryRunner::Selection#call refers to env more than
    self (FeatureEnvy)

After:

 1: NoBrainer::QueryRunner::Selection has no descriptive comment
    (IrresponsibleModule)

Notes:

Extracted class method normalize_query_and_selection, which returns
the normalized env hash. The awesome side-effect is that #call is now
a one-liner :)