MAINT: Remove conda from Appveyor script

[matthew-brett]

We can get away without conda, now wheels are available.

Simplify file otherwise, by removing unnecessary quotes and backslashes.

[effigies]

LGTM, if it passes.

[coveralls]

Coverage remained the same at 96.255% when pulling f292f59 on matthew-brett:python-org-appveyor into d20388b on nipy:master.

[codecov-io]

Codecov Report

Merging #584 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #584   +/-   ##
=======================================
  Coverage   94.32%   94.32%           
=======================================
  Files         177      177           
  Lines       24697    24697           
  Branches     2640     2640           
=======================================
  Hits        23295    23295           
  Misses        925      925           
  Partials      477      477

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