Fix the AppVeyor build

[sharwell]

• Remove the build configuration element, which was preventing the build_script section from being recognized
• Only build the master branch (and pull requests sent to it)

[codecov-io]

Codecov Report

Merging #34 into master will not change coverage. The diff coverage is n/a.

@@          Coverage Diff           @@
##           master     #34   +/-   ##
======================================
  Coverage    61.9%   61.9%           
======================================
  Files         371     371           
  Lines       18386   18386           
  Branches     3499    3499           
======================================
  Hits        11382   11382           
  Misses       5747    5747           
  Partials     1257    1257

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