Replace iOS7orLower boolean with isIOS7

Element.GetClientRects seems to be incorrect only on iOS 7.

Closes #264

interact.js

@@ -321,10 +321,9 @@
             supportsTouch &&
             navigator.userAgent.match('Presto'),
 
-        // scrolling doesn't change the result of
-        // getBoundingClientRect/getClientRects on iOS <=7 but it does on iOS 8
-        isIOS7orLower = (/iP(hone|od|ad)/.test(navigator.platform)
-                            && /OS [1-7][^\d]/.test(navigator.appVersion)),
+        // scrolling doesn't change the result of getClientRects on iOS 7
+        isIOS7 = (/iP(hone|od|ad)/.test(navigator.platform)
+                         && /OS 7[^\d]/.test(navigator.appVersion)),
 
         // prefix matchesSelector
         prefixedMatchesSelector = 'matches' in Element.prototype?
@@ -714,7 +713,7 @@
     function getElementRect (element) {
         var clientRect = getElementClientRect(element);
 
-        if (!isIOS7orLower && clientRect) {
+        if (!isIOS7 && clientRect) {
             var scroll = getScrollXY(getWindow(element));
 
             clientRect.left   += scroll.x;