Point to secure github repo.

[csgillespie]

The drat repo supports https. Since CRAN is https, makes sense to switch.

[eddelbuettel]

Don't disagree, but we may need to depend on a minimum version of R in order to ensure consistent https client-side support.

[csgillespie]

Drat already depends on R (>= 3.2.0). So we should be OK in terms of dependencies.

[eddelbuettel]

Also in ChangeLog:

2015-02-02  Dirk Eddelbuettel  <edd@debian.org>

    * R/addRepo.R (addRepo): Use http, not https, at GitHub

    * man/drat-package.Rd: Ditto

So yes, this just completes things. Thanks for the catch!