gopkg: Use version instead of branch

[gaocegege]

Ref https://github.com/golang/dep/blob/master/docs/Gopkg.toml.md#version

Note: When you specify a version without an operator, dep automatically uses the ^ operator by default. dep ensure will interpret the given version as the min-boundary of a range.

Using version allows us to be compatible with more versions.

/hold

/cc @zjj2wry @codeflitting Does it work for you?

Signed-off-by: Ce Gao [email protected]

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This change is 

[gaocegege]

/assign @zjj2wry

[coveralls]

Coverage remained the same at 64.496% when pulling c843614 on gaocegege:1.10 into 3a0efc1 on kubeflow:master.

[gaocegege]

/retest

[zjj2wry]

branches may change all the time, using version ensures that the code for dep ensure is consistent.
/lgtm

[zjj2wry]

/assign @jlewi

[gaocegege]

/approve
/hold cancel

[k8s-ci-robot]

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