Add Cairo as required package

In reference to issue #5
diff --git a/.Rproj.user/441ADA46/sources/prop/60EAAA4E b/.Rproj.user/441ADA46/sources/prop/60EAAA4E
index 9960a89..c8a6372 100644
--- a/.Rproj.user/441ADA46/sources/prop/60EAAA4E
+++ b/.Rproj.user/441ADA46/sources/prop/60EAAA4E
@@ -1,4 +1,4 @@
 {
-    "cursorPosition" : "4,131",
-    "scrollLine" : "0"
+    "cursorPosition" : "32,0",
+    "scrollLine" : "26"
 }
\ No newline at end of file