Replace import path 'Sirupsen/logrus' with 'sirupsen/logrus'

[muesli]

Use the new canonical project URL (note the lowercase 's') for logrus to prevent duplicate checkouts & imports of logrus.

See: sirupsen/logrus#384

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[muesli]

I signed it!

[googlebot]

CLAs look good, thanks!

[sdboyer]

hah! funny timing.

yes, we're definitely aware of the problem - see #1079, sirupsen/logrus#451 (comment). this actual instance of Sirupsen has no risk of being harmful, as it's not a real import statement. but still, it doesn't hurt us to send all the right signals!