Fix week and month Time grain in MySQL

[prihoda]

With mysql datetime and timestamp columns, currently the Time grain "week" and "month" options don't remove the time part. This results in groupings like this:

timestamp count
2015-04-05 07:00:00 1
2015-04-05 10:00:00 1
2015-04-05 11:00:00 2
2015-04-05 11:50:00 1
2015-04-05 12:00:00 5
2015-04-05 14:20:00 1
2015-04-05 14:30:00 1

and so on.

This is solved by wrapping the DATE_SUB with DATE().

[landscape-bot]

Repository health decreased by 0.04% when pulling 41c7766 on prihoda:patch-1 into 481d821 on airbnb:master.

• 1 new problem was found (including 0 errors and 0 code smells).
• No problems were fixed.

[coveralls]

Coverage remained the same at 71.177% when pulling 41c7766 on prihoda:patch-1 into 481d821 on airbnb:airbnb_prod.

[landscape-bot]

Code quality remained the same when pulling 37e72f3 on prihoda:patch-1 into 481d821 on airbnb:master.

[coveralls]

Coverage remained the same at 71.177% when pulling 37e72f3 on prihoda:patch-1 into 481d821 on airbnb:airbnb_prod.

[prihoda]

Sorry, wrong branch, will try to be more careful next time.