Update vendored dependencies

This includes github.com/kurin/blazer 0.2.0, which resolves #1291

vendor/github.com/kurin/blazer/b2/readerat.go

@@ -0,0 +1,48 @@
+// Copyright 2017, Google
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package b2
+
+import (
+	"io"
+	"sync"
+)
+
+type readerAt struct {
+	rs io.ReadSeeker
+	mu sync.Mutex
+}
+
+func (r *readerAt) ReadAt(p []byte, off int64) (int, error) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+
+	// ReadAt is supposed to preserve the offset.
+	cur, err := r.rs.Seek(0, io.SeekCurrent)
+	if err != nil {
+		return 0, err
+	}
+	defer func() { r.rs.Seek(cur, io.SeekStart) }()
+
+	if _, err := r.rs.Seek(off, io.SeekStart); err != nil {
+		return 0, err
+	}
+	return io.ReadFull(r.rs, p)
+}
+
+// wraps a ReadSeeker in a mutex to provite a ReaderAt how is this not in the
+// io package?
+func enReaderAt(rs io.ReadSeeker) io.ReaderAt {
+	return &readerAt{rs: rs}
+}