ci: Require go.mod to be tidy

[rhcarvalho]

This helps us prevent accidentally having an inconsistent go.mod file
after changes to dependencies.

[rhcarvalho]

Waiting to see a Travis failure -- currently our go.mod is not tidy.
Will update the PR after confirming the .travis.yml config is good to catch future cases of untidiness :)

[rhcarvalho]

Travis job: https://travis-ci.com/getsentry/sentry-go/builds/141893202

• Builds with GOMODULE111=on failed as expected (go.mod not tidy) except for master, in which go mod tidy didn't change go.mod but found a data race in TestNewDsn. (update: this is not a data race in sentry-go, but a problem in github.com/google/go-cmp, see Bump github.com/google/go-cmp to 0.4.0 (required for Go 1.14) #129)
• Builds with GOMODULE111=off failed because it is wrong to run go mod tidy when modules are disabled.

TODO:

• Do not run go mod tidy when GOMODULE111=off.

[rhcarvalho]

Travis job: https://travis-ci.com/getsentry/sentry-go/builds/141935681

• GO111MODULE=off: all green.
• GO111MODULE=on: Go 1.11, 1.12 and 1.13 red because master has a go.mod tidy issue -- fail as expected (or is it go mod tidy that has an issue? Seems likely for Go < 1.14).
  Go@master fails because of a test dependency -- go mod tidy did not change go.mod. Looks like its behavior is going to change in Go 1.14.
  I think the difference is attributable to cmd/go: 'mod tidy' chooses points in requirement cycles that differ from other subcommands golang/go#34086. In fact, with Go 1.13, running go mod tidy in sentry-go@master changes go.mod and then running go build reverts the change!
  Changing go.mod to match what go mod tidy wants would break the build/tests:

$ go mod tidy
$ git diff
diff --git a/go.mod b/go.mod
index a5a446f..ed7a7b2 100644
--- a/go.mod
+++ b/go.mod
@@ -22,7 +22,7 @@ require (
        github.com/pkg/errors v0.8.1
        github.com/sergi/go-diff v1.0.0 // indirect
        github.com/smartystreets/goconvey v1.6.4 // indirect
-       github.com/ugorji/go v1.1.7 // indirect
+       github.com/ugorji/go/codec v1.1.7 // indirect
        github.com/urfave/negroni v1.0.0
        github.com/valyala/fasthttp v1.6.0
        github.com/xeipuuv/gojsonschema v1.2.0 // indirect
$ GOFLAGS=-mod=readonly go build
go: updates to go.mod needed, disabled by -mod=readonly
$ GOFLAGS=-mod=readonly go test
go: updates to go.mod needed, disabled by -mod=readonly

[rhcarvalho]

Once Go 1.14 gets released, its go mod tidy will possibly disagree with 1.13 and earlier.
That would make running go mod tidy in CI awkward.

Putting this on hold to evaluate the situation better.

[rhcarvalho]

Revived this.

We can have a check for "tidiness" as long as we agree on a single authoritative Go version to use.

One day this might become part of the go command, see https://togithub.com/golang/go/issues/27005.