Run tests with Github Actions

[pdecat]

This PR adds support for running tests with Github Actions as the Travis CI job does not seem to be enabled.

[pdecat]

Github Actions test workflow is now working and picking up build errors because mongodb dependencies introduced by #53 are not vendored:

pagerduty/cache.go:13:2: cannot find package "go.mongodb.org/mongo-driver/bson" in any of:
	/home/runner/work/go-pagerduty/go-pagerduty/src/github.com/heimweh/go-pagerduty/vendor/go.mongodb.org/mongo-driver/bson (vendor tree)
	/opt/hostedtoolcache/go/1.16.7/x64/src/go.mongodb.org/mongo-driver/bson (from $GOROOT)
	/home/runner/work/go-pagerduty/go-pagerduty/src/go.mongodb.org/mongo-driver/bson (from $GOPATH)
pagerduty/cache.go:14:2: cannot find package "go.mongodb.org/mongo-driver/bson/primitive" in any of:
	/home/runner/work/go-pagerduty/go-pagerduty/src/github.com/heimweh/go-pagerduty/vendor/go.mongodb.org/mongo-driver/bson/primitive (vendor tree)
	/opt/hostedtoolcache/go/1.16.7/x64/src/go.mongodb.org/mongo-driver/bson/primitive (from $GOROOT)
	/home/runner/work/go-pagerduty/go-pagerduty/src/go.mongodb.org/mongo-driver/bson/primitive (from $GOPATH)
pagerduty/cache.go:15:2: cannot find package "go.mongodb.org/mongo-driver/mongo" in any of:
	/home/runner/work/go-pagerduty/go-pagerduty/src/github.com/heimweh/go-pagerduty/vendor/go.mongodb.org/mongo-driver/mongo (vendor tree)
	/opt/hostedtoolcache/go/1.16.7/x64/src/go.mongodb.org/mongo-driver/mongo (from $GOROOT)
	/home/runner/work/go-pagerduty/go-pagerduty/src/go.mongodb.org/mongo-driver/mongo (from $GOPATH)
pagerduty/cache.go:16:2: cannot find package "go.mongodb.org/mongo-driver/mongo/options" in any of:
	/home/runner/work/go-pagerduty/go-pagerduty/src/github.com/heimweh/go-pagerduty/vendor/go.mongodb.org/mongo-driver/mongo/options (vendor tree)
	/opt/hostedtoolcache/go/1.16.7/x64/src/go.mongodb.org/mongo-driver/mongo/options (from $GOROOT)
	/home/runner/work/go-pagerduty/go-pagerduty/src/go.mongodb.org/mongo-driver/mongo/options (from $GOPATH)
pagerduty/cache.go:17:2: cannot find package "go.mongodb.org/mongo-driver/mongo/readpref" in any of:
	/home/runner/work/go-pagerduty/go-pagerduty/src/github.com/heimweh/go-pagerduty/vendor/go.mongodb.org/mongo-driver/mongo/readpref (vendor tree)
	/opt/hostedtoolcache/go/1.16.7/x64/src/go.mongodb.org/mongo-driver/mongo/readpref (from $GOROOT)
	/home/runner/work/go-pagerduty/go-pagerduty/src/go.mongodb.org/mongo-driver/mongo/readpref (from $GOPATH)

https://github.com/pdecat/go-pagerduty/runs/3430412065?check_suite_focus=true#step:4:17

Also, what about switching from govendor to go modules to manage dependencies?

[pdecat]

See actions/setup-go#102 (comment) for why this is needed.

[heimweh]

Hey @pdecat! This looks great 🎉

Let's merge this one and switch to go modules while we're at it so we can fix the dep issues 👍