Invalid build files in dash-renderer build directory

[T4rk1n]

Running the karma tests in dash-renderer (cd dash/dash-renderer && npm run test) create build files in dash/dash-renderer/build which contains the renderer distribution, these files should not be distributed along the package and fails the postbuild escheck.

Could add a clean command before the build or change the karma config to output it's build in another directory.

[alexcjohnson]

Moving the karma output seems cleaner, then we can gitignore that dir entirely.