gnu: Use HTTPS for r-project.org home pages.
* gnu/packages/cran.scm (r-performanceanalytics, r-qap, r-gclus, r-leaps)
(r-splus2r, r-ifultools, r-sapa, r-quantmod, r-tseries, r-wmtsa, r-fractal)
(r-urca, r-trend, r-expm, r-complexplus, r-powerplus, r-signal, r-abind)
(r-prroc)[home-page]: Use HTTPS.
* gnu/packages/emacs.scm (emacs-ess)[home-page]: Likewise.
* gnu/packages/graph.scm (r-diffusionmap)[home-page]: Likewise.
* gnu/packages/statistics.scm (r-minimal, r-quantreg)[home-page]: Likewise.