Fix DATA-DOG/go-sqlmock dependency

[unguiculus]

Running go mod tidy results in the following error:

go mod tidy
go: finding gopkg.in/DATA-DOG/go-sqlmock.v1 v1.3.3
github.com/myuser/myapp/pkg/health imports
	github.com/InVisionApp/go-health/checkers tested by
	github.com/InVisionApp/go-health/checkers.test imports
	gopkg.in/DATA-DOG/go-sqlmock.v1: cannot find module providing package gopkg.in/DATA-DOG/go-sqlmock.v1

This PR should fix the dependency.

See also for further reference:

• v1.3.1 Breaks build DATA-DOG/go-sqlmock#161
• 23 fix problem with DATA-DOG/go-sqlmock vmware-archive/healthcheck#24

[rhoetsc]

@schigh Can you have a look, please.

[schigh]

Yessir. Apologies for not seeing these alerts sooner!

[schigh]

👍 LGTM 🌮 🌮 🌮 🌮 🌮

[schigh]

@unguiculus @rhoetsc I am following this up by moving this entire repo to modules very soon (today-ish)

[unguiculus]

@schigh Thanks for getting this in so quickly. 🎉

I had to fix the module migration, though. See #70 and #71.