simplify the build script run on appveyor

[shiftkey]

PowerShell v2 requires .NET 2 to be installed, so let's see if we can simplify the build script and address #253 at the same time.

Resolves #253

[shiftkey]

This seems to have done the trick:

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Package-Squirrel
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Executing task: Package-Squirrel
Creating directory C:/projects/carnac/deploy/Squirrel
Attempting to build package from 'Carnac.temp.nuspec'.
Successfully created package 'C:\projects\carnac\deploy\Squirrel\carnac.2.3.13-minimal-script-s0001.nupkg'.
Finished executing task: Package-Squirrel
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