Add Cairo as required package

In reference to issue #5
diff --git a/.Rproj.user/441ADA46/pcs/windowlayoutstate.pper b/.Rproj.user/441ADA46/pcs/windowlayoutstate.pper
index 4373672..e9b88cf 100644
--- a/.Rproj.user/441ADA46/pcs/windowlayoutstate.pper
+++ b/.Rproj.user/441ADA46/pcs/windowlayoutstate.pper
@@ -2,7 +2,7 @@
     "left" : {
         "panelheight" : 789,
         "splitterpos" : 329,
-        "topwindowstate" : "EXCLUSIVE",
+        "topwindowstate" : "NORMAL",
         "windowheight" : 828
     },
     "right" : {
diff --git a/.Rproj.user/441ADA46/pcs/workbench-pane.pper b/.Rproj.user/441ADA46/pcs/workbench-pane.pper
index d5d6c04..b15bf23 100644
--- a/.Rproj.user/441ADA46/pcs/workbench-pane.pper
+++ b/.Rproj.user/441ADA46/pcs/workbench-pane.pper
@@ -2,7 +2,5 @@
     "TabSet1" : 3,
     "TabSet2" : 0,
     "TabZoom" : {
-        "MaximizedTab" : "Source",
-        "WidgetSize" : 502
     }
 }
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