increase the version requirement of rmarkdown as 1.6 just released
diff --git a/DESCRIPTION b/DESCRIPTION
index 86caaf4..b8e17b7 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -27,7 +27,7 @@
     stringr (>= 1.0),
     xml2,
     rvest,
-    rmarkdown (>= 1.4.0),
+    rmarkdown (>= 1.6.0),
     readr
 Suggests:
     testthat