Fixed mysql has gone away error by disabling innodb_file_per_table option.

This changes the default to disable innodb_file_per_table option because by default on OSX there is a limit of 256 open files per process, which breaks tests for larger projects that use awesome mrd gem.

The detailed description of this problem is here: https://github.com/mezis/mrd/issues/8

@mezis Already agreed to accept this pull request in the corresponding issue - thank you! I'm not sure about bumping the version of the gem.