Add DT to imports

[hadley]

Since you almost always need it

[jimhester]

Except you don't need it at all when using covr on travis and DT is a fairly heavy package. @jennybc was actually the person who prompted me to move it to Suggests, back when reporting relied on shiny (r-lib/covr#53 (comment))

There have been a number of issues related to this, r-lib/covr#269, r-lib/covr#343 etc. I may improve the error message when DT is not installed, but I think having it in Suggests is still correct.

[hadley]

Hmmm, good points. Maybe we should add to devtools’ imports?

[jimhester]

Yeah, we could probably do that.

[hadley]

This problem struck again in the class I'm teaching today.

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