vendor: Replace Sirupsen/logrus with sirupsen/logrus

With:

  $ git mv vendor/github.com/{S,s}irupsen
  $ sed -i 's/Sirupsen/sirupsen/g' $(git grep -l Sirupsen)

catching up with the upstream lowercasing [1,2,3,4].  Because of the
compatibility issues discussed in [3], some consumers may prefer to
use the old uppercase version until they have time to update their
other Logrus consumers to the new lowercase form.

[1]: https://github.com/sirupsen/logrus/blame/v1.0.3/README.md#L6
[2]: https://github.com/sirupsen/logrus/pull/384
[3]: https://github.com/sirupsen/logrus/issues/570#issuecomment-313933276
[4]: https://github.com/sirupsen/logrus/issues/553

vendor/github.com/sirupsen/logrus/.travis.yml

@@ -0,0 +1,15 @@
+language: go
+go:
+  - 1.6.x
+  - 1.7.x
+  - 1.8.x
+  - tip
+env:
+  - GOMAXPROCS=4 GORACE=halt_on_error=1
+install:
+  - go get github.com/stretchr/testify/assert
+  - go get gopkg.in/gemnasium/logrus-airbrake-hook.v2
+  - go get golang.org/x/sys/unix
+  - go get golang.org/x/sys/windows
+script:
+  - go test -race -v ./...