Revert to sirupsen -> Sirupsen

[evalphobia]

As discussed on the issue #8, using import path both of github.com/logrus/sirupsen and github.com/logrus/Sirupsen might occur case-insensitive import collision error...

The name changing was reverted on sirupsen/logrus@2680936 .

I checked hooks for logrus, most of them including official Syslog hook use github.com/logrus/Sirupsen, so I would like to standardize the naming to Sirupsen to avoid the error . 💓

[evalphobia]

Oh, thank you for quick action 😹

[polds]

Thank you :)