Fixing header plugin

[bsipocz]

This causes an error that overshadows failures with v4.0rc1 testing

[MSeifert04]

This means we need to drop astropy 2.x support immediately?

Or is there a way we can avoid this exception (I'm not as familiar with the test-infrastructure as I once was):

Copying ccdproc.egg-info to /tmp/ccdproc-test-5e_vamb3/lib/python3.7/site-packages/ccdproc-2.1.dev284-py3.7.egg-info
running install_scripts
Traceback (most recent call last):
  File "/home/travis/miniconda/envs/test/lib/python3.7/site-packages/_pytest/config/__init__.py", line 372, in _getconftestmodules
    return self._path2confmods[path]
KeyError: local('/tmp/ccdproc-test-5e_vamb3/lib/python3.7/site-packages/ccdproc')
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
  File "/home/travis/miniconda/envs/test/lib/python3.7/site-packages/_pytest/config/__init__.py", line 403, in _importconftest
    return self._conftestpath2mod[conftestpath]
KeyError: local('/tmp/ccdproc-test-5e_vamb3/lib/python3.7/site-packages/ccdproc/conftest.py')
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
  File "/home/travis/miniconda/envs/test/lib/python3.7/site-packages/_pytest/config/__init__.py", line 409, in _importconftest
    mod = conftestpath.pyimport()
  File "/home/travis/miniconda/envs/test/lib/python3.7/site-packages/py/_path/local.py", line 701, in pyimport
    __import__(modname)
  File "/home/travis/miniconda/envs/test/lib/python3.7/site-packages/_pytest/assertion/rewrite.py", line 226, in load_module
    py.builtin.exec_(co, mod.__dict__)
  File "/tmp/ccdproc-test-5e_vamb3/lib/python3.7/site-packages/ccdproc/conftest.py", line 9, in <module>
    from pytest_astropy_header.display import (PYTEST_HEADER_MODULES,
ModuleNotFoundError: No module named 'pytest_astropy_header'
ERROR: could not load /tmp/ccdproc-test-5e_vamb3/lib/python3.7/site-packages/ccdproc/conftest.py
The command "$MAIN_CMD $SETUP_CMD" exited with 4.

[MSeifert04]

Since I don't know if and how long we want to keep support for astropy 2.x I decided to cherry-pick the commit here in #710 and keep it astropy 2.x compatible for now.