Sets first letter of all sirupsen imports lower-case

[stweiz]

This pull request sets all import paths and other references of "sirupsen" to lower case letters, so everything should be unified.

[sirupsen]

This is going to break Logrus from too many people yet. We'll have to create a new syslog package, preferably out of repo, that imports lower-case.

[schultz9999]

We have a patch to go thru sources and do the manual patch once we import to vendor dir :-/ Kinda painful.

@find vendor/ -name '*.go' | xargs gofmt -w -r '"github.com/Sirupsen/logrus" -> "github.com/sirupsen/logrus"'

[schultz9999]

If it's a breaking change, why not to release a major version?

[sirupsen]

This was done in master with a bump of a major version.