Merge pull request #120 from caspervdw/display-multichannel

Display multichannel image
FIX: Travis build

as in https://github.com/soft-matter/trackpy/pull/186
Remove rm libm.so.6

It now breaks at this line. Maybe libm.so.6 is fixed in 3.7.3, so try
without.
Merge pull request #126 from caspervdw/fix-build

FIX: Travis build
Add natural_keys method
Add natural sorted image list to ImageSequence
Remove import in __init__.py