Downgrade govendor to fix mongodb-proxy compilation

While deploying the mongodb_node_ng the mongodb_proxy fails to compile:
```
+ set -e
+ cp -a services/tools/mongodb_proxy/README.md services/tools/mongodb_proxy/build.sh services/tools/mongodb_proxy/config services/tools/mongodb_proxy/src /var/vcap/pack
ages/mongodb_proxy
+ cp -a services/govendor /var/vcap/packages/mongodb_proxy
++ readlink -nf /var/vcap/packages/golang
+ GOLANG_PATH=/var/vcap/data/packages/golang/0.1-dev.1
+ PATH=/var/vcap/data/packages/golang/0.1-dev.1/bin:/var/vcap/bosh/bin:/usr/local/bin:/usr/local/sbin:/bin:/sbin:/usr/bin:/usr/sbin:/usr/X11R6/bin
++ readlink -nf /var/vcap/packages/golang
+ export GOROOT=/var/vcap/data/packages/golang/0.1-dev.1
+ GOROOT=/var/vcap/data/packages/golang/0.1-dev.1
++ readlink -nf /var/vcap/packages/mongodb_proxy/govendor
+ GOVENDOR=/var/vcap/data/packages/mongodb_proxy/0.1-dev.1/govendor
++ readlink -nf /var/vcap/packages/mongodb_proxy
+ export GOPATH=/var/vcap/data/packages/mongodb_proxy/0.1-dev.1:/var/vcap/data/packages/mongodb_proxy/0.1-dev.1/govendor
+ GOPATH=/var/vcap/data/packages/mongodb_proxy/0.1-dev.1:/var/vcap/data/packages/mongodb_proxy/0.1-dev.1/govendor
+ cd /var/vcap/packages/mongodb_proxy/src
+ go install proxyctl
# launchpad.net/goyaml
decode.go: In function '_cgo_bec77eaae85a_Cfunc_event_alias':
decode.go:34: warning: assignment from incompatible pointer type
decode.go: In function '_cgo_bec77eaae85a_Cfunc_event_mapping_start':
decode.go:44: warning: assignment from incompatible pointer type
decode.go: In function '_cgo_bec77eaae85a_Cfunc_event_scalar':
decode.go:54: warning: assignment from incompatible pointer type
decode.go: In function '_cgo_bec77eaae85a_Cfunc_event_sequence_start':
decode.go:64: warning: assignment from incompatible pointer type
# go-mongo-proxy/proxy
go-mongo-proxy/proxy/server.go:144: cannot convert nil to type steno.Logger
```

After some digging a found that govendor was upgraded when mongodb was moved to the contrib repo.
cf-services was on cf852275a2014f517dce7a68dec03f0c38f336e2 as can be seen [here](https://github.com/cloudfoundry/cf-release/tree/1a54a39cde05089b0a83982011e35239c987170b/src).
And govendor was on 6c5ff67e8aacc2847dab3239001fa2513af54558 as can be seen [here](https://github.com/cloudfoundry/vcap-services/tree/cf852275a2014f517dce7a68dec03f0c38f336e2)