fix(mongoose): Use connection.readyState to check DB

[BrunnerLivio]

This uses a public API which is supported in both monog < 4 and up.
Fixes issue #207

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: #207

What is the new behavior?

Tested with 3.4. Uses the connection.readyState API which is available in all MongoDB versions

Does this PR introduce a breaking change?

[ ] Yes
[ ] No

Other information