Add Feedlr::Error::ServiceUnavailableError

To support the case below.
cursor=#<Feedlr::Base errorCode=503 errorId="ap7-sv2.2014101600.3589028" errorMessage="too many requests, try again later">
Merge pull request #2 from y-ken/patch-1

Add Feedlr::Error::ServiceUnavailableError
Ruby >= 1.9.3 is needed now
update travis settings and README to reflect usage of Ruby >= 1.9.3