fix mysql < 5.7.5 with only_full_group_by

From MySQL's docs, versions less than 5.7.5 support SQL-92's version of
GROUP BY, which requires all non-aggregate columns to show up in the
GROUP BY.

> SQL:1999 and later permits such nonaggregates per optional feature T301
> if they are functionally dependent on GROUP BY columns: If such
> a relationship exists between name and custid, the query is legal. This
> would be the case, for example, were custid a primary key of customers.
> MySQL 5.7.5 and later implements detection of functional dependence.
> https://dev.mysql.com/doc/refman/5.7/en/group-by-handling.html

Turns out Travis hasn't been testing on 5.6, which explains why it was
working (to my bewilderment, as I know this is an issue).

Thanks @augensalat for reporting this issue!

Fixes #36

.travis.yml

@@ -28,7 +28,6 @@ matrix:
         env: MYSQL_VERSION=5.7 MYSQL_STRICT=1
       - perl: "5.28"
         env: MYSQL_VERSION=5.6 MYSQL_STRICT=1
-        dist: trusty
       - perl: "5.28"
         env: MYSQL_VERSION=8.0 MYSQL_UTF8MB4=1
       - perl: "5.28"
@@ -47,7 +46,7 @@ before_install:
         . /etc/lsb-release;
         sudo add-apt-repository "deb http://repo.mysql.com/apt/ubuntu/ $DISTRIB_CODENAME mysql-$MYSQL_VERSION";
         sudo apt-get update;
-        sudo DEBIAN_FRONTEND=noninteractive apt-get -q --yes --fix-broken --allow-unauthenticated --option DPkg::Options::=--force-confnew install mysql-server;
+        sudo DEBIAN_FRONTEND=noninteractive apt-get -q -t mysql-$MYSQL_VERSION --yes --fix-broken --allow-unauthenticated --option DPkg::Options::=--force-confnew install mysql-server;
         echo "upgrading mysql";
         sudo mysql_upgrade -u root --password='' --force;
         sudo service mysql restart;

lib/Minion/Backend/mysql.pm

@@ -603,7 +603,7 @@ sub _try {
       AND job.queue IN ($qq) AND job.task IN ($qt)
       $sql_job_id
       AND (job.expires IS NULL OR job.expires > NOW())
-    GROUP BY job.id
+    GROUP BY job.id, job.args, job.retries, job.task
     HAVING SUM(
       parent.state IS NOT NULL AND (
         parent.state = 'active'