Properly dispatch by api version in server mode

I'm not happy with this solution. Please think of a fix if you're reading this.

moto/elb/urls.py

@@ -1,10 +1,38 @@
 from __future__ import unicode_literals
 from .responses import ELBResponse
+from moto.elbv2.responses import ELBV2Response
+
+
+def api_version_elb_backend(*args, **kwargs):
+    """
+    ELB and ELBV2 (Classic and Application load balancers) use the same
+    hostname and url space. To differentiate them we must read the
+    `Version` parameter out of the url-encoded request body. TODO: There
+    has _got_ to be a better way to do this. Please help us think of
+    one.
+    """
+    request = args[0]
+
+    if hasattr(request, 'values'):
+        # boto3
+        version = request.values.get('Version')
+    else:
+        # boto
+        request.parse_request()
+        version = request.querystring.get('Version')[0]
+
+    if '2012-06-01' == version:
+        return ELBResponse.dispatch(*args, **kwargs)
+    elif '2015-12-01' == version:
+        return ELBV2Response.dispatch(*args, **kwargs)
+    else:
+        raise Exception("Unknown ELB API version: {}".format(version))
+
 
 url_bases = [
     "https?://elasticloadbalancing.(.+).amazonaws.com",
 ]
 
 url_paths = {
-    '{0}/$': ELBResponse.dispatch,
+    '{0}/$': api_version_elb_backend,
 }