dep ensure报错unable to update checked out version: fatal: reference is not a tree:

dep ensure报如下错误：

(1) ✗ unable to update checked out version: fatal: reference is not a tree: 2b758aa73013d2b1f11077aeca87ea042a1438bf
(1) : command failed: [git checkout 2b758aa73013d2b1f11077aeca87ea042a1438bf]: exit status 128
(1) ← no more versions of github.com/bradfitz/slice to try; begin backtrack

但是查看过 github.com/bradfitz/slice ，2b758aa73013d2b1f11077aeca87ea042a1438bf命名是master上的最新一条commit。几经尝试和搜索，找到了如下可行的解决方法：

删除 $GOPATH/pkg/dep/sources

参考： https://github.com/golang/dep/issues/699