gnu: r-formatr: Update to 1.8.

* gnu/packages/statistics.scm (r-formatr): Update to 1.8.
[native-inputs]: Add r-knitr.

gnu/packages/statistics.scm

@@ -1329,14 +1329,16 @@ adapted for other output formats, such as HTML or LaTeX.")
 (define-public r-formatr
   (package
     (name "r-formatr")
-    (version "1.7")
+    (version "1.8")
     (source (origin
               (method url-fetch)
               (uri (cran-uri "formatR" version))
               (sha256
                (base32
-                "1nsxbrx31k3y6yql30qkrvdfyznlia2qfvwv95mfiy7m7wdn4rm3"))))
+                "04q6y53ji7ll131khbwiz584wl8sinhh3z82p6r8dqa9f618ay6z"))))
     (build-system r-build-system)
+    (native-inputs
+     `(("r-knitr" ,r-knitr)))
     (home-page "https://yihui.org/formatr/")
     (synopsis "Format R code automatically")
     (description