Updated to mysql2 version 0.3.12b6.

This contains an important bugfix, so that timeouts of c mysql are correctly used. More details see https://github.com/brianmario/mysql2/pull/287 .

This is an issue with AWS RDS, which once in a while delays queries up to 15 minutes. Details see https://forums.aws.amazon.com/thread.jspa?messageID=446622&tstart=0https://forums.aws.amazon.com/thread.jspa?threadID=122020&tstart=0 and https://forums.aws.amazon.com/thread.jspa?threadID=122035&tstart=0

Bugfix requested by @anne-schulz .

I would suggest to release a new version with version number `2.0.12b6` to reflect the usage of a beta version.