Fix exposure cube test on Python 2.6

I see this error from `astropy.io.fits` with Python 2.6 :
https://gist.github.com/cdeil/0875c51b495f51066f2e

The same error can be seen on travis-ci on Linux:
https://travis-ci.org/gammapy/gammapy/jobs/30449578#L623

We'll probably have to wait for https://github.com/astropy/astropy/issues/2774 to fix this and until the fix is in a stable version before we can merge this PR.