use latin1 in case of mysql

[punchdrunker]

this PR is for #83
I fixed migration error under the condition of using mysql and utf8.

Mysql2::Error: Specified key was too long; max key length is 767 bytes:
CREATE UNIQUE INDEX `read_marks_reader_readable_index`  ON `read_marks` (`reader_id`, `reader_type`, `readable_type`, `readable_id`) 

[coveralls]

Coverage remained the same at 100.0% when pulling 7cf752f on punchdrunker:work/use-latin1 into 186cfdc on ledermann:master.

[ledermann]

Thanks!