fix: add @types/ dev dependencies for tsc. fixes #340

[guyellis]

I tested this by:

• npm link the connect-mongo repo with the current master checked out.
• npm link connect-mongo on a dependent TypeScript project
• Run tsc on dependent project and confirm errors seen in Compilation of a typescript project with connect-mongo dependency fails #340
• Add @types to connect-mongo
• Run tsc on dependent project and confirm no errors.

[coveralls]

Coverage remained the same at 81.649% when pulling 286d1e7 on guyellis:master into cbc95be on jdesboeufs:master.

[mingchuno]

@guyellis should you run yarn install so that the lock file got updated?

[guyellis]

@mingchuno - yes - good call - I was wondering why the npm lockfile had been excluded in gitignore and now I see the project is using yarn. I've updated it and pushed.