Set ValueError .args attribute, for Python 2.7

This should hopefully fix up our CI results.

jplephem/exceptions.py

@@ -4,14 +4,13 @@ class OutOfRangeError(ValueError):
     """One or more time values given were out of range for the ephemeris.
 
     This exception is thrown if any input times are out of the range of
-    times supported by an ephemeris.  It has two extra attributes:
+    times supported by an ephemeris.  It has an extra attribute:
 
-    - `message`: a string explaining what happened
     - `out_of_range_times`: if the input `tdb` of times is an array,
       this provides an array of booleans of the same length where `True`
       means the corresponding date is out of range.
 
     """
     def __init__(self, message, out_of_range_times):
-        self.message = message
+        self.args = message,
         self.out_of_range_times = out_of_range_times