Fix AppVeyor build script to correctly return error code

The script chained the execution of VsDevCmd.bat (aka the "Developer
Command Prompt") with build.cmd which seems to confuse AppVeyor
into not recognizing a non-zero exit code (i.e. when a test fails)
and incorrectly marking the build as green.

Invoking VsDevCmd.bat and build.cmd in two separate steps fixes this.

Fixes #214