Add projected completion date for epics

[E70M]

Exposes the projected_completion attribute from the Pivotal API on the epic resource

Usage

client = TrackerApi::Client.new(token: token)
project = client.project(project_id)
epics = project.epics(fields: ":default,followers,projected_completion")
projected_completion = epics.first.projected_completion

Documentation

https://www.pivotaltracker.com/help/api/rest/v5#epic_resource

[codeclimate]

Code Climate has analyzed commit 1d08a05 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (50% is the threshold).

This pull request will bring the total coverage in the repository to 97.5% (0.0% change).

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[irphilli]

This looks great, thanks!