Merge pull request #185 from matthew-brett/refactor-mgh-image

RF: changes to MGH image to standarize API

Remove get_affine method in favor of base class
Remove get_header method in favor of base class
Refactor update_header to put code in base class
Added __eq__ and get_slope_inter to MGHHeader for compatibility with Nifti etc API.
BF: update travis build to use https for pypi

It looks like pypi disabled http package access in favor of https;
update the .travis.yml build to use https instead.