Update phantomscript.js

Computing the display size seems to cause the svg rendering to complete bofore saving the image. Referencing issue #181 https://github.com/knsv/mermaid/issues/181

lib/phantomscript.js

@@ -256,6 +256,10 @@ function executeInPage(data) {
   svg = document.querySelector('svg')
   svgValue = xmlSerializer.serializeToString(svg)
 
+  // make sure the entire view area is calculated before exiting
+  // this solves an issue where the svg is not done rendering when the image is created
+  svg.getBBox();
+
   //console.log(document.body.outerHTML);
 
   return svgValue