Bobby warnings should be reported at WARN level

[howyp]

Bobby seems to log everything at INFO level, even if it's a warning. This seems surprising and also makes it harder to parse the warnings out of the log. Example:

[info] [bobby] Checking for out of date dependencies
[info] [bobby] 'scala-library 2.11.2' is out of date, consider upgrading to '2.11.5'
[info] [bobby] Unable to get a latestRelease number for 'com.kenshoo:metrics-play:2.3.0_0.1.6'
[info] [bobby] 'play-reactivemongo 3.1.0' is out of date, consider upgrading to '3.2.0'

I'd suggest that this should have been:

[info] [bobby] Checking for out of date dependencies
[warn] [bobby] 'scala-library 2.11.2' is out of date, consider upgrading to '2.11.5'
[info] [bobby] Unable to get a latestRelease number for 'com.kenshoo:metrics-play:2.3.0_0.1.6'
[warn] [bobby] 'play-reactivemongo 3.1.0' is out of date, consider upgrading to '3.2.0'

Not sure about what level we should log when a version number couldn't be retrieved.

[SteveSmithTech]

@howyp Would you like to submit a PR?

[charleskubicek]

I'm not sure being a one patch version out in a dependency should be a warning. i.e 1.0.4 vs 1.0.5 shouldn't be such a big deal. I'd rather save warnings for larger version differences.

[howyp]

Working out what should be a warning was kind of the reason for raising an issue rather than a PR. We wanted to use bobby warnings to make CI go yellow.

[howyp]

What do you think the threshold for giving a warning should be?