changed ordering of `brightness_temperature` arguments; AstropyDeprec…

[privong]

When testing examples from the docs, I received the following warning when computing the Jy/K conversion factor:

In [1]: from radio_beam import Beam                                                           

In [2]: import astropy.units as u                                                             

In [3]: mb = Beam(60*u.arcsec)                                                                

In [4]: (1*u.Jy).to(u.K, u.brightness_temperature(mb, 115 * u.GHz))                           
WARNING: AstropyDeprecationWarning: The inputs to `brightness_temperature` have changed. Frequency is now the first input, and angular area is the second, optional input. [astropy.units.equivalencies]
Out[4]: <Quantity 0.0256694 K>

I have astropy 4.0.1 installed:

In [9]: import astropy                                                                        

In [10]: astropy.__version__                                                                  
Out[10]: '4.0.1'

[keflavich]

travis failures are unrelated.