Adapted to work in Mysql 8

[Redithion]

Added version to decide how to get the current isolation level. In MySQL <= 5.7 is tx_isolation, in MySQL 8 is transaction_isolation

[Redithion]

I forgot to update the version, might be worth doing so.

[mathieujobin]

Interesting... although I am using this gem with MySQL 8 and I did not encounter a problem...