Fix postgresql.rollbacks metric and don't crash when tags are None.

postgresql.rollbacks metric was collecting the wrong data due to a typo.
chef-datadog 1.0.1 can create a tags entry with a None value, causing the
the agent to crash. This is now fixed.
Info command now return non-zero when errors occur
(this includes checks.d errors)
Add jenkins metrics fix #567
Merge pull request #557 from DataDog/postgres-none-tags

Fix postgresql.rollbacks metric and don't crash when tags are None.
save EC2 metadata between lookups so that if a lookup ever times out we still have the old value. fixes #554.
Collect MySQL InnoDB metrics only if InnoDB is enabled, fix #566
Merge pull request #565 from DataDog/info-non-zero

Info command now return non-zero when errors occur
Remove nginx assertion

fixes #569
Skip memory tests on travis to unbreak the build
Merge branch 'master' into dogstatsd-autorestart