Bump aws sdk to v1.15.11

This is the latest official release for this dependency

Signed-off-by: Ryan Abrams <rdabrams@gmail.com>

vendor/github.com/aws/aws-sdk-go/internal/sdkio/io_go1.6.go

@@ -0,0 +1,10 @@
+// +build !go1.7
+
+package sdkio
+
+// Copy of Go 1.7 io package's Seeker constants.
+const (
+	SeekStart   = 0 // seek relative to the origin of the file
+	SeekCurrent = 1 // seek relative to the current offset
+	SeekEnd     = 2 // seek relative to the end
+)

vendor/github.com/aws/aws-sdk-go/internal/sdkio/io_go1.7.go

@@ -0,0 +1,12 @@
+// +build go1.7
+
+package sdkio
+
+import "io"
+
+// Alias for Go 1.7 io package Seeker constants
+const (
+	SeekStart   = io.SeekStart   // seek relative to the origin of the file
+	SeekCurrent = io.SeekCurrent // seek relative to the current offset
+	SeekEnd     = io.SeekEnd     // seek relative to the end
+)