Revert "handle regex special characters"

This reverts commit 8dd8345.

We shouldn't have changed the original behavior, which
too many people are relying on at this point.

It's also technically a breaking change, which we shouldn't
have landed on a minor/patch version change. So in the interest
of not breaking people's logs in production, reverting this.

We can discuss in a new issue if we want to restore this patch
for a `v3` release, but at the moment I'm personally leaningo
towards *no*, for historical reasons (i.e. this is reminding
me of Node.js trying to remove `sys` if anybody reading this
remembers those days).

See the discussion in #250 for more backlog.

debug.js

@@ -144,7 +144,7 @@ function enable(namespaces) {
 
   for (var i = 0; i < len; i++) {
     if (!split[i]) continue; // ignore empty strings
-    namespaces = split[i].replace(/[\\^$+?.()|[\]{}]/g, '\\$&').replace(/\*/g, '.*?');
+    namespaces = split[i].replace(/\*/g, '.*?');
     if (namespaces[0] === '-') {
       exports.skips.push(new RegExp('^' + namespaces.substr(1) + '$'));
     } else {