Fix date strings passed to influx

Date strings must be wrapped in single quotes

metobsapi/util/query_influx.py

@@ -11,7 +11,7 @@ def parse_dt(d):
     elif isinstance(d, timedelta):
         return 'now() - {:d}s'.format(int(d.total_seconds()))
     else:
-        return d.strftime('%Y-%m-%dT%H:%M:%SZ')
+        return d.strftime('\'%Y-%m-%dT%H:%M:%SZ\'')
 
 
 def build_queries(symbols, begin, end, value):