Add Cairo as required package

In reference to issue #5
diff --git a/.Rproj.user/441ADA46/sources/prop/B68389EB b/.Rproj.user/441ADA46/sources/prop/B68389EB
index 6519c24..1c34024 100644
--- a/.Rproj.user/441ADA46/sources/prop/B68389EB
+++ b/.Rproj.user/441ADA46/sources/prop/B68389EB
@@ -1,4 +1,4 @@
 {
-    "cursorPosition" : "41,0",
-    "scrollLine" : "0"
+    "cursorPosition" : "83,65",
+    "scrollLine" : "76"
 }
\ No newline at end of file