fix fibermap dtype for astropy 1.3

[sbailey]

This PR works around a change in behavior from astropy 1.2 to 1.3, likely related to the units issue we had to work around desihub/specsim#62:

Under astropy 1.2, assigning a value to a table column would retain the original dtype of that column. Under astropy 1.3, the column gets the new dtype of the right-hand-side value. This breaks some consistency checks for fibermap columns though I don't think it broke any actual functionality (for desispec).

This PR also fixes two smaller issues that I found while debugging this:

• previously desispec.io.Brick couldn't be created with a filename that didn't include a path (it would parse the path as '' and then try to create '' and complain)
• typo in new fibermap columns LOCATION,DEVICE_LOC,PETAL_LOC #379 (new fibermap columns) resulted in both LAMBDAREF and LAMBDA_REF

I have tested with astropy 1.3 on edison and astropy 1.2 on my laptop. I'm Leaving .travis.yml at astropy 1.2.1 for now; the nightly cronjob unit tests on edison test master with astropy 1.3.

[sbailey]

I'm going to go ahead and merge this now to get tests passing again under astropy 1.3 at NERSC, but feel free to make retroactive comments if you find something wrong.