Add Cairo as required package

In reference to issue #5
diff --git a/DESCRIPTION b/DESCRIPTION
index 9b27e89..23d1dda 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -14,5 +14,6 @@
     dplyr,
     ggplot2,
     rmarkdown,
+    Cairo,
     knitr
 RoxygenNote: 6.1.0.9000