Update several R packages (r-shiny prereqs)

r-jsonlite 0.0.21 -> 1.0
r-mime     0.4    -> 0.5
rcpp       0.12.5 -> 0.12.6

CRAN is funny.  The older versions of these packages are still available
in package specific directories but the current version is not there, so
I don't see any way to make the older versions work.

var/spack/repos/builtin/packages/r-rcpp/package.py

@@ -37,10 +37,10 @@ class RRcpp(Package):
     last two."""
 
     homepage = "http://dirk.eddelbuettel.com/code/rcpp.html"
-    url      = "https://cran.r-project.org/src/contrib/Rcpp_0.12.5.tar.gz"
+    url      = "https://cran.r-project.org/src/contrib/Rcpp_0.12.6.tar.gz"
     list_url = "https://cran.r-project.org/src/contrib/Archive/Rcpp"
 
-    version('0.12.5', 'f03ec05b4e391cc46e7ce330e82ff5e2')
+    version('0.12.6', 'db4280fb0a79cd19be73a662c33b0a8b')
 
     extends('R')