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/hooks.go

@@ -0,0 +1,34 @@
+package logrus
+
+// A hook to be fired when logging on the logging levels returned from
+// `Levels()` on your implementation of the interface. Note that this is not
+// fired in a goroutine or a channel with workers, you should handle such
+// functionality yourself if your call is non-blocking and you don't wish for
+// the logging calls for levels returned from `Levels()` to block.
+type Hook interface {
+	Levels() []Level
+	Fire(*Entry) error
+}
+
+// Internal type for storing the hooks on a logger instance.
+type LevelHooks map[Level][]Hook
+
+// Add a hook to an instance of logger. This is called with
+// `log.Hooks.Add(new(MyHook))` where `MyHook` implements the `Hook` interface.
+func (hooks LevelHooks) Add(hook Hook) {
+	for _, level := range hook.Levels() {
+		hooks[level] = append(hooks[level], hook)
+	}
+}
+
+// Fire all the hooks for the passed level. Used by `entry.log` to fire
+// appropriate hooks for a log entry.
+func (hooks LevelHooks) Fire(level Level, entry *Entry) error {
+	for _, hook := range hooks[level] {
+		if err := hook.Fire(entry); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}