Rename package from logrus_airbrake to airbrake

Using underscores in package names in discouraged:
https://golang.org/doc/effective_go.html#package-names

Given that this package is in a subdirectory of the logrus package,
the name `airbrake` should be sufficiently descriptive.
diff --git a/hooks/airbrake/airbrake.go b/hooks/airbrake/airbrake.go
index 75f4db1..9fa108a 100644
--- a/hooks/airbrake/airbrake.go
+++ b/hooks/airbrake/airbrake.go
@@ -1,4 +1,4 @@
-package logrus_airbrake
+package airbrake
 
 import (
 	"github.com/Sirupsen/logrus"
diff --git a/hooks/airbrake/airbrake_test.go b/hooks/airbrake/airbrake_test.go
index d2fd61d..55df200 100644
--- a/hooks/airbrake/airbrake_test.go
+++ b/hooks/airbrake/airbrake_test.go
@@ -1,4 +1,4 @@
-package logrus_airbrake
+package airbrake
 
 import (
 	"encoding/xml"