Added cairoVersion test

Readme.md

@@ -11,6 +11,8 @@
 
     $ npm install canvas
 
+If not previously installed, you will want to install the [cairo graphics library](http://cairographics.org/download/) first using the package manager available to you, or building from source.
+
 ## Example
 
     var Canvas = require('canvas')

test/canvas.test.js

@@ -29,6 +29,10 @@ module.exports = {
     assert.match(Canvas.version, /^\d+\.\d+\.\d+$/);
   },
   
+  'test .cairoVersion': function(assert){
+    assert.match(Canvas.cairoVersion, /^\d+\.\d+\.\d+$/);
+  },
+  
   'test .parseColor()': function(assert){
     assert.equal(null, Canvas.parseColor());
     assert.equal(null, Canvas.parseColor(''));