MAINT, CI: Travis rackcdn->conda.org
MAINT, CI: Travis rackcdn->conda.org

* transition to usage of anaconda.org for retrieving
NumPy pre-release ("nightly") wheels in Travis CI, because
continued dependence on rackcdn will eventually lead to charges
for the OSS community

* the `NUMPYSPEC` lines for pre-release matrix entries were modified
based on the instructions (https://anaconda.org/scipy-wheels-nightly/numpy)
and local testing:

* 1) Use the new URL

* 2) Switch from `-f` (finding archives/tarballs at a URL) to `-i`
(use a PEP 503 compliant index like anaconda.org `simple` API format)

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