Use simplejson for better error descriptions.

The standard json library only provides an error
that the json failed to parse.
simplejson provides actual character and line numbers
along with an expected character in it's errors.
This makes writing elasticsearch queries much easier.

redash/query_runner/elasticsearch.py

@@ -1,4 +1,4 @@
-import json
+import simplejson as json
 import logging
 import sys
 import urllib

requirements.txt

@@ -41,3 +41,4 @@ pystache==0.5.4
 parsedatetime==2.1
 cryptography==1.4
 Flask-Limiter==0.9.3
+simplejson==3.10.0