Building on Windows fails because of rm in pre-build script

[hannesne]

Building on Windows fails because of rm shell command in pre-build script. NPM on Windows still uses cmd.exe to execute commands. rm is only available in PowerShell and Bash. The build step fails with 'rm' is not recognized as an internal or external command, operable program or batch file. Happening in Dev branch at the moment. Will create a PR with a fix.

[hannesne]

It would also be a good idea to add a Windows-based build to the Travis pipelines, to prevent this kind of issue in future.