(Re-targeted version of #490)

This PR sets the MySQL sql_mode to a strict setting for our tests.

We do this in two ways/places:

• Set it globally for Travis-CI, since we want all travis tests to be strict and we don't care about messing with the environment since its temporary anyway
• Set it for the session when testing locally, so that it doesn't change your local MySQL configuration semi-permanently (which may be used for other local DB's)

Setting the sql_mode in this way should make our testing more reliable and will most likely even restore the seemingly vanished bug described in #438 (that was originally fixed in #317 and then again in #440).