Remove uneeded @ which may output error messages

This PR removes what seems like unneeded `@` characters. Some of them produce an error (even though the script runs fine):

```dos
F:\tmp\beat>bin\beat install
'@set' is not recognized as an internal or external command,
operable program or batch file.
F:\tmp\beat\erts-6.2\bin\erlsrv.exe: Service beat_0.1.0 added to system.
```
@nuex Can you fetch this PR and test on your side ?