Refactored smells out of selection/order_by.rb

Smells reported by reek:

Before:

 3: NoBrainer::Selection::OrderBy#order_by calls rules[0] twice
    (DuplicateMethodCall)
 5: NoBrainer::Selection::OrderBy#order_by has the variable name 'k'
    (UncommunicativeVariableName)
 5: NoBrainer::Selection::OrderBy#order_by has the variable name 'v'
    (UncommunicativeVariableName)

After:

Nothing to report

Notes:

Fairly trivial change to the uncommunicative variable names and a
local variable to track rules[0]