Meteor 3.0 compatible

[p-wieser]

• Bump versions requirements
• Use Async form of server API
• replace Future with Promise

[p-wieser]

Oop's.. Sorry, this is an error of mine to create a single PR for all these things :(
Actually there is here:

• what I believe be the minimal update to be compatible with Meteor 3.0 (the first three commits)
• three extensions that I have needed to login with zimbra.
  Sorry for the inconvenience and thank you for your comprehension.

PS: I have an accounts-zimbra package ready to be published, but waiting for yours. So, you would be very kind to merge and publish. :)

[JackAdams]

Thank you for all your work on this. I'm pinned to 2.x for the foreseeable future. Would you consider making a fork of this package and being the 3.x maintainer?

If you're okay with that, I'll update the README.md to explain, with a link to your version for 3.x

[p-wieser]

Hello,
I can be wrong, but I believe that Async API and Promises are available starting with Meteor 2.8. If true, then the patches I propose would be compatible with 2.8+. Have to be tested of course.
I think this might be a better option to keep a single repository and prevent future double maintenance of the code ?
But, yes, I'am willing to help with Meteor 3.x issues.
If you nonetheless prefer your first option, your package has been forked into https://github.com/p-wieser/meteor-accounts-ldap.
Regards
Pierre