fix MySQL replication status aquisition bug

[takaidohigasi]

WHY

mackerel-plugin-mysql fails on replica status aquisition (MySQL major version is not related)
#7

WHAT

fix replica status aquisition bug ( to work with current show slave status format: 1 row response)

NOTE

• show slave status returns only 1 row (which has multiple columns) but former implementation assume it is multiple rows like show status query
• there isn't test for replica metric (also for this test) but I confirmed the test.sh on my localhost

taka-h@H37R9YYYGV mackerel-plugin-mysql % bash test.sh
cba1da4cf1213b79f8bdd5cc9e394c2e55f7ef2324b23c656cfd7d4e94125594
Connecting 1...
test-mackerel-plugin-mysql
test-mackerel-plugin-mysql
taka-h@H37R9YYYGV mackerel-plugin-mysql % echo $?
0

[takaidohigasi]

@yseto would you please take a look when you have time?

[lufia]

I will review this pull request ASAP.

[lufia]

@takaidohigasi Thanks! We will release new version in the next few days.