Please support go mod

[skyrocknroll]

Support Go mod so that downstream projects can migrate to go mod

[skyrocknroll]

jaegertracing/jaeger-client-go#382

[yurishkuro]

For the time being, we need to keep Gopkg.toml, so if we add support for go mod, we need to manually keep it in sync w/ Gopkg.toml and add a new build to the Travis matrix.

[sagikazarmark]

Currently this project is not installable on Go 1.13 when a project uses go modules:

	go.uber.org/cadence/client imports
	go.uber.org/cadence/internal tested by
	go.uber.org/cadence/internal.test imports
	github.com/uber/jaeger-client-go/config tested by
	github.com/uber/jaeger-client-go/config.test imports
	github.com/uber/jaeger-lib/metrics/testutils: module github.com/uber/jaeger-lib@latest (v2.0.0+incompatible) found, but does not contain package github.com/uber/jaeger-lib/metrics/testutils

Actually, the root cause is github.com/uber/jaeger-client-go not using go modules. Working on the syntax that uses an earlier version that still works.

[sagikazarmark]

Turns out upgrading the transitive dependency is sufficient:

github.com/uber/jaeger-client-go v2.17.0+incompatible // indirect