update Gopkg.toml, use new version of dep, update vendored files

...hub.com/prometheus/client_golang/prometheus/gauge_test.go

.../prometheus/client_golang/prometheus/go_collector_test.go

...com/prometheus/client_golang/prometheus/histogram_test.go

...thub.com/prometheus/client_golang/prometheus/http_test.go

...ub.com/prometheus/client_golang/prometheus/metric_test.go

...etheus/client_golang/prometheus/process_collector_test.go

...prometheus/client_golang/prometheus/promhttp/http_test.go

....com/prometheus/client_golang/prometheus/registry_test.go

...b.com/prometheus/client_golang/prometheus/summary_test.go

...ithub.com/prometheus/client_golang/prometheus/vec_test.go

vendor/github.com/prometheus/client_model/.gitignore

vendor/github.com/prometheus/client_model/CONTRIBUTING.md

vendor/github.com/prometheus/client_model/MAINTAINERS.md

vendor/github.com/prometheus/client_model/Makefile

vendor/github.com/prometheus/client_model/README.md

vendor/github.com/prometheus/client_model/metrics.proto

vendor/github.com/prometheus/client_model/pom.xml

vendor/github.com/prometheus/client_model/ruby/LICENSE

vendor/github.com/prometheus/client_model/setup.py

vendor/github.com/prometheus/common/.travis.yml