internal: remove usage of x/net/context (#57)

diff --git a/invoke.go b/invoke.go
index 86049d8..cb5cd2a 100644
--- a/invoke.go
+++ b/invoke.go
@@ -30,9 +30,8 @@
 package gax
 
 import (
+	"context"
 	"time"
-
-	"golang.org/x/net/context"
 )
 
 // A user defined call stub.
diff --git a/invoke_test.go b/invoke_test.go
index 3d12e60..d476d14 100644
--- a/invoke_test.go
+++ b/invoke_test.go
@@ -30,11 +30,10 @@
 package gax
 
 import (
+	"context"
 	"errors"
 	"testing"
 	"time"
-
-	"golang.org/x/net/context"
 )
 
 var canceledContext context.Context