Tweak an error message

I have encountered the following error message in a real world
application development.

```console
RuntimeError: Runtime log file 'tmp/parallel_runtime_rspec.log' does not
contain sufficient data to sort 578 test files, please update it.
```

I think that removing a runtime log file may be the faster solution.
This PR adds a hint to options users should choose.

lib/parallel_tests/test/runner.rb

@@ -173,7 +173,7 @@ def sort_by_runtime(tests, runtimes, options={})
             allowed_missing -= 1 unless time = runtimes[test]
             if allowed_missing < 0
               log = options[:runtime_log] || runtime_log
-              raise "Runtime log file '#{log}' does not contain sufficient data to sort #{tests.size} test files, please update it."
+              raise "Runtime log file '#{log}' does not contain sufficient data to sort #{tests.size} test files, please update or remove it."
             end
             [test, time]
           end