update gemspec to support bundler 1.14

Refactors Inbox, deprecates JRuby, other fixes

Summary:
- removes `Inbox` in favor of `Nylas`
- Simplifies gemspec and fixes nylas#84
- Removes JRuby support

Send draft version when deleting a draft

Fixes nylas#87

Revert "Merge branch 'mguymon-jruby_support'"

This reverts commit 89c59a7, reversing
changes made to e2067ab.

Test Plan: Ran tests

Reviewers: khamidou, bengotow

Differential Revision: https://phab.nylas.com/D3397

Regenerate gemspec for version 2.0.1

Regenerate gemspec for version 2.0.0

Regenerate gemspec for version 1.3.0

Regenerate gemspec for version 1.2.1

update CHANGELOG for release 1.2.0

Regenerate gemspec for version 1.1.0

Regenerate gemspec for version 1.0.0

Regenerate gemspec for version 0.18.2