gnu: Add r-devtools.

* gnu/packages/statistics.scm (r-devtools): New variable.

gnu/packages/statistics.scm

@@ -1235,3 +1235,32 @@ (define-public r-rstudioapi
      "This package provides functions to access the RStudio API and provide
 informative error messages when it's not available.")
     (license license:expat)))
+
+(define-public r-devtools
+  (package
+    (name "r-devtools")
+    (version "1.9.1")
+    (source (origin
+              (method url-fetch)
+              (uri (cran-uri "devtools" version))
+              (sha256
+               (base32
+                "10ycx3kkiz5x8nmgw31d9wa5hhlx2fhda2nqzxfrczqpz1jik6ci"))))
+    (build-system r-build-system)
+    (propagated-inputs
+     `(("r-curl" ,r-curl)
+       ("r-digest" ,r-digest)
+       ("r-evaluate" ,r-evaluate)
+       ("r-git2r" ,r-git2r)
+       ("r-httr" ,r-httr)
+       ("r-jsonlite" ,r-jsonlite)
+       ("r-memoise" ,r-memoise)
+       ("r-roxygen2" ,r-roxygen2)
+       ("r-rstudioapi" ,r-rstudioapi)
+       ("r-rversions" ,r-rversions)
+       ("r-whisker" ,r-whisker)))
+    (home-page "https://github.com/hadley/devtools")
+    (synopsis "Tools to make developing R packages easier")
+    (description "The devtools package is a collection of package development
+tools to simplify the devolpment of R packages.")
+    (license license:gpl2+)))