Fix crash when parsing previous version resultset.json

[PragTob]

We changed our file format with the 0.18.0 release series - unfortunately our code still seems to want to read the old file format and then break ;) We gotta be more careful there. I don't think we need to be able to read all old versions of files but I think we should rather capture the error and continue without processing the old results (easiest to achieve I believe).

This is my preliminary diagnosis, problems goes away when deleting the coverage directory.
Reported by @klyonrad #781 (comment)

bundle show simplecov
/Users/username/.rvm/gems/ruby-2.6.5/gems/simplecov-0.18.0.beta1

bundle show simplecov-html
/Users/username/.rvm/gems/ruby-2.6.5/gems/simplecov-html-0.11.0.beta1

bundle exec rspec spec/policies
Run options: exclude {:future=>true}                                                                                 

Randomized with seed 25469
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Finished in 23.37 seconds (files took 10.5 seconds to load)
237 examples, 0 failures

Randomized with seed 25469
Traceback (most recent call last):
	15: from /Users/username/.rvm/gems/ruby-2.6.5/gems/simplecov-0.18.0.beta1/lib/simplecov/defaults.rb:29:in `block in <top (required)>'
	14: from /Users/username/.rvm/gems/ruby-2.6.5/gems/simplecov-0.18.0.beta1/lib/simplecov.rb:164:in `run_exit_tasks!'
	13: from /Users/username/.rvm/gems/ruby-2.6.5/gems/simplecov-0.18.0.beta1/lib/simplecov/configuration.rb:196:in `block in at_exit'
	12: from /Users/username/.rvm/gems/ruby-2.6.5/gems/simplecov-0.18.0.beta1/lib/simplecov.rb:74:in `result'
	11: from /Users/username/.rvm/gems/ruby-2.6.5/gems/simplecov-0.18.0.beta1/lib/simplecov/result_merger.rb:83:in `merged_result'
	10: from /Users/username/.rvm/gems/ruby-2.6.5/gems/simplecov-0.18.0.beta1/lib/simplecov/result_merger.rb:57:in `results'
	 9: from /Users/username/.rvm/gems/ruby-2.6.5/gems/simplecov-0.18.0.beta1/lib/simplecov/result_merger.rb:57:in `each'
	 8: from /Users/username/.rvm/gems/ruby-2.6.5/gems/simplecov-0.18.0.beta1/lib/simplecov/result_merger.rb:58:in `block in results'
	 7: from /Users/username/.rvm/gems/ruby-2.6.5/gems/simplecov-0.18.0.beta1/lib/simplecov/result.rb:72:in `from_hash'
	 6: from /Users/username/.rvm/gems/ruby-2.6.5/gems/simplecov-0.18.0.beta1/lib/simplecov/result.rb:86:in `symbolize_names_of_coverage_results'
	 5: from /Users/username/.rvm/gems/ruby-2.6.5/gems/simplecov-0.18.0.beta1/lib/simplecov/result.rb:86:in `each_with_object'
	 4: from /Users/username/.rvm/gems/ruby-2.6.5/gems/simplecov-0.18.0.beta1/lib/simplecov/result.rb:86:in `each'
	 3: from /Users/username/.rvm/gems/ruby-2.6.5/gems/simplecov-0.18.0.beta1/lib/simplecov/result.rb:87:in `block in symbolize_names_of_coverage_results'
	 2: from /Users/username/.rvm/gems/ruby-2.6.5/gems/simplecov-0.18.0.beta1/lib/simplecov/result.rb:87:in `each_with_object'
	 1: from /Users/username/.rvm/gems/ruby-2.6.5/gems/simplecov-0.18.0.beta1/lib/simplecov/result.rb:87:in `each'
/Users/username/.rvm/gems/ruby-2.6.5/gems/simplecov-0.18.0.beta1/lib/simplecov/result.rb:88:in `block (2 levels) in symbolize_names_of_coverage_results': undefined method `to_sym' for 1:Integer (NoMethodError)
Did you mean?  to_s

#781

[PragTob]

fix-crash-against-old-resultset-json has a scenario aimed at reproducing this.

Right now it's incomplete because of:

     if use_merging
        wait_for_other_processes
        SimpleCov::ResultMerger.store_result(@result) if result?
        @result = SimpleCov::ResultMerger.merged_result
      end

If store_result is executed before the result merging then this bug doesn't trigger. So gotta make result? be false in the scenario :)

[PragTob]

WIP fix/reproduction is up at #824

[PragTob]

Fixed in #824