DOC: update conda quickerstart guide to use environment.yml
DOC: update conda quickerstart guide to use environment.yml

Also add better installer options, and remove the unnecessary
XCode command line tools.

mdhaber/scipy,scipy/scipy,perimosocordiae/scipy,anntzer/scipy,matthew-brett/scipy,perimosocordiae/scipy,zerothi/scipy,grlee77/scipy,ilayn/scipy,e-q/scipy,perimosocordiae/scipy,anntzer/scipy,e-q/scipy,Stefan-Endres/scipy,grlee77/scipy,e-q/scipy,WarrenWeckesser/scipy,WarrenWeckesser/scipy,mdhaber/scipy,perimosocordiae/scipy,WarrenWeckesser/scipy,Stefan-Endres/scipy,matthew-brett/scipy,zerothi/scipy,rgommers/scipy,Eric89GXL/scipy,andyfaff/scipy,ilayn/scipy,vigna/scipy,Stefan-Endres/scipy,matthew-brett/scipy,Eric89GXL/scipy,anntzer/scipy,anntzer/scipy,anntzer/scipy,tylerjereddy/scipy,zerothi/scipy,scipy/scipy,endolith/scipy,ilayn/scipy,Stefan-Endres/scipy,ilayn/scipy,Eric89GXL/scipy,e-q/scipy,WarrenWeckesser/scipy,grlee77/scipy,Eric89GXL/scipy,tylerjereddy/scipy,zerothi/scipy,rgommers/scipy,mdhaber/scipy,andyfaff/scipy,endolith/scipy,andyfaff/scipy,WarrenWeckesser/scipy,tylerjereddy/scipy,ilayn/scipy,perimosocordiae/scipy,matthew-brett/scipy,endolith/scipy,andyfaff/scipy,tylerjereddy/scipy,tylerjereddy/scipy,zerothi/scipy,Eric89GXL/scipy,rgommers/scipy,mdhaber/scipy,endolith/scipy,andyfaff/scipy,rgommers/scipy,endolith/scipy,grlee77/scipy,mdhaber/scipy,perimosocordiae/scipy,anntzer/scipy,vigna/scipy,vigna/scipy,vigna/scipy,mdhaber/scipy,matthew-brett/scipy,scipy/scipy,scipy/scipy,ilayn/scipy,endolith/scipy,Eric89GXL/scipy,scipy/scipy,grlee77/scipy,scipy/scipy,e-q/scipy,Stefan-Endres/scipy,zerothi/scipy,WarrenWeckesser/scipy,vigna/scipy,rgommers/scipy,andyfaff/scipy,Stefan-Endres/scipy