Gemfile: Update Berkshelf to version 6 (fix travis)
Gemfile: Update Berkshelf to version 6 (fix travis)

zuazo/mysql_tuning-cookbook,zuazo/mysql_tuning-cookbook,onddo/mysql_tuning-cookbook,onddo/mysql_tuning-cookbook,onddo/mysql_tuning-cookbook,zuazo/mysql_tuning-cookbook