Add Cairo as required package

In reference to issue #5
diff --git a/.Rproj.user/441ADA46/sources/prop/AE6C9F23 b/.Rproj.user/441ADA46/sources/prop/AE6C9F23
index 7a73a41..7d1ed6b 100644
--- a/.Rproj.user/441ADA46/sources/prop/AE6C9F23
+++ b/.Rproj.user/441ADA46/sources/prop/AE6C9F23
@@ -1,2 +1,4 @@
 {
+    "cursorPosition" : "15,14",
+    "scrollLine" : "0"
 }
\ No newline at end of file