Bugfix maximum coverage drop with old .last_run file

Author: petertellgren

features/maximum_coverage_drop.feature

@@ -145,6 +145,7 @@ Feature:
       require 'simplecov'
       SimpleCov.start do
         add_filter 'test.rb'
+        maximum_coverage_drop 0
       end
       """
     And the file "coverage/.last_run.json" with:

lib/simplecov/exit_codes/maximum_coverage_drop_check.rb

@@ -62,7 +62,7 @@ def compute_coverage_drop_data
       def last_coverage(criterion)
         last_coverage_percent = last_run[:result][criterion]
 
-        if !last_coverage_percent && criterion == "line"
+        if !last_coverage_percent && criterion == :line
           last_run[:result][:covered_percent]
         else
           last_coverage_percent