[BUG] Changes to astropy.convolution.Gaussian2DKernel break tests

[sfarrens]

System setup
OS: [e.g] macOS v10.14.1
Python version: [e.g.] v3.6.7
Python environment (if any): [e.g.] conda v4.5.11

Describe the bug
ModOpt unit test test_gaussian_kernel_none is broken as the Astropy Gaussian2DKernel method has been updated in v5.2 (see changelog)

To Reproduce
Run unit tests with Astropy installed.

Are you planning to submit a Pull Request?

• Yes
• No