Simplify travis, scipy instead of iminuit as default

[JelleAalbers]

This wil:

• Set scipy minimizers to be the default, even if you have iminuit installed. In my code and what I've seen from @kdund we use scipy minimizers exclusively; apparently the problems we once had with these are now resolved and/or we had other problems with iminuit. Or maybe it's just setting the Powell method that did the magic? In any case, if nobody is using iminuit it probably shouldn't be the default. @pelssers, since you implemented the iminuit minimizers, would you agree?
• Simplify the travis build instructions. Since wheels exist we do not need anaconda to install binaries, so the travis file can just be a few lines.
• Also test python 3.6 on Travis.

[pelssers]

I agree with using scipy as default, it is the more robust option and well tested in the xenon analysis.

[JelleAalbers]

Ok, thanks!