Fix astropy.utils.OrderedDict deprecation

A fix for the warning: `WARNING: AstropyDeprecationWarning: astropy.utils.compat.odict.OrderedDict is now deprecated - import OrderedDict from the collections module instead [astropy.utils.compat.odict]`

specutils/wcs/specwcs.py

@@ -10,7 +10,7 @@
 from ..models.BSplineModel import BSplineModel
 import astropy.units as u
 
-from astropy.utils import OrderedDict
+from collections import OrderedDict
 from astropy.io import fits
 import copy
 from astropy import constants