build.ps1 - Validate sha1 hash for files downloaded from spotify build server.

[amaitland]

The tar.bz2 files have a sha1 hash that we should calculate and compare against to validate the files were downloaded successfully and abort the build if failure.

The sha1 hash can be obtained from the same json file we get the urls from (http://opensource.spotify.com/cefbuilds/index.json)

[amaitland]

Done