UTF-8-encode input from log files in stats reporting

This fixes issue [#7763](https://issues.dp.la/issues/7763)

`get-api-stats.rb` was throwing an ArgumentError at line 80 if the line being read in from the log file had a non-UTF-8 character.  This change encodes each log file line as UTF-8 before the regex evaluation.  It also ensures that the regex will use UTF-8 (line 67).