Refactor towards a for loop and .push while removing redundant check

```unshift``` is significantly slower than ```push```. Shift towards
using ```push``` and remove a redundant conditional.

Author: johnbacon

src/raven.js

@@ -642,15 +642,15 @@ function isSetup() {
 function joinRegExp(patterns) {
     // Combine an array of regular expressions and strings into one large regexp
     // Be mad.
-    var sources = [], i = patterns.length;
-    while (i--) {
+    var sources = [];
+    for (var i = 0; i < patterns.length; i++) {
         if (isString(patterns[i])) {
             // If it's a string, we need to escape it
             // Taken from: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions
-            sources.unshift(patterns[i].replace(/([.*+?^=!:${}()|\[\]\/\\])/g, "\\$1"));
-        } else if (!isUndefined(patterns[i]) && patterns[i] && patterns[i].source) {
+            sources.push(patterns[i].replace(/([.*+?^=!:${}()|\[\]\/\\])/g, "\\$1"));
+        } else if (patterns[i] && patterns[i].source) {
             // If it's a regexp already, we want to extract the source
-            sources.unshift(patterns[i].source);
+            sources.push(patterns[i].source);
         }
         // Intentionally skip other cases
     }