TST: Change most travisci tests to Python3.6.
TST: Change most travisci tests to Python3.6.

Python 3.6 is more current and needs more testing than it is currently
getting. This is also useful preparation for droppping 2.7.

ahaldane/numpy,madphysicist/numpy,WarrenWeckesser/numpy,jakirkham/numpy,simongibbons/numpy,rgommers/numpy,endolith/numpy,grlee77/numpy,pdebuyl/numpy,shoyer/numpy,seberg/numpy,ahaldane/numpy,tynn/numpy,endolith/numpy,mattip/numpy,MSeifert04/numpy,grlee77/numpy,pizzathief/numpy,pdebuyl/numpy,jorisvandenbossche/numpy,jorisvandenbossche/numpy,tynn/numpy,rgommers/numpy,pbrod/numpy,mhvk/numpy,mattip/numpy,WarrenWeckesser/numpy,ahaldane/numpy,Eric89GXL/numpy,WarrenWeckesser/numpy,grlee77/numpy,shoyer/numpy,charris/numpy,gfyoung/numpy,anntzer/numpy,madphysicist/numpy,simongibbons/numpy,numpy/numpy,abalkin/numpy,charris/numpy,pbrod/numpy,mhvk/numpy,simongibbons/numpy,jakirkham/numpy,gfyoung/numpy,mhvk/numpy,grlee77/numpy,pizzathief/numpy,ahaldane/numpy,ahaldane/numpy,simongibbons/numpy,MSeifert04/numpy,rgommers/numpy,numpy/numpy,endolith/numpy,endolith/numpy,pbrod/numpy,mattip/numpy,WarrenWeckesser/numpy,charris/numpy,seberg/numpy,anntzer/numpy,pdebuyl/numpy,jorisvandenbossche/numpy,shoyer/numpy,Eric89GXL/numpy,tynn/numpy,simongibbons/numpy,Eric89GXL/numpy,numpy/numpy,madphysicist/numpy,jakirkham/numpy,madphysicist/numpy,gfyoung/numpy,seberg/numpy,MSeifert04/numpy,madphysicist/numpy,pizzathief/numpy,jakirkham/numpy,pizzathief/numpy,shoyer/numpy,anntzer/numpy,Eric89GXL/numpy,WarrenWeckesser/numpy,MSeifert04/numpy,pbrod/numpy,seberg/numpy,abalkin/numpy,jorisvandenbossche/numpy,anntzer/numpy,jakirkham/numpy,mattip/numpy,pizzathief/numpy,shoyer/numpy,pdebuyl/numpy,rgommers/numpy,jorisvandenbossche/numpy,MSeifert04/numpy,abalkin/numpy,grlee77/numpy,numpy/numpy,charris/numpy,mhvk/numpy,pbrod/numpy,mhvk/numpy