fix browser detection for jQuery > 2.0

jQuery removed $.support.leadingWhitespace from jquery greater 2.0
because support was dropped for IE 7/8. So if property is undefined,
we are pretty shure its ie > 8 or jQuery does not support the browser
anyway.

src/jquery.flot.js

@@ -361,7 +361,7 @@ Licensed under the MIT license.
                     c = sin.toFixed(6),     // when we add them to the string
                     transformRule;
 
-                if ($.support.leadingWhitespace) {
+                if (!($.support.leadingWhitespace === false)) {
 
                     // The transform origin defaults to '50% 50%', producing
                     // blurry text on some browsers (Chrome) when the width or