recursively create cache directory

This should fix r-lib#60. As discussed in the links from that issue, by
default ESS turns on caching, which causes lintr to try to create a
subdirectory inside of ~/.R. However, if ~/.R doesn't exist in the
first place, dir.create will fail and lintr will not work in emacs. By
adding the recursive = TRUE argument to dir.create, this will now
create all parent directories as necessary to create the desired cache.

R/cache.R

@@ -51,7 +51,7 @@ save_cache <- function(cache, file, path = NULL) {
   }
 
   if (!file.exists(path)) {
-    dir.create(path)
+    dir.create(path, recursive = TRUE)
   }
 
   save(file = get_cache_file_path(file, path), envir = cache, list = ls(envir = cache))