force symlink of netdata-updater.sh

[mrdrogdrog]

Today I moved the netdata git into another directory and executed the installer to refresh the files with the new path. But the script failed when it tried to (re)create the symlink for the cronjob. This fix just adds the force flag to ln.

[CLAassistant]

All committers have signed the CLA.

[ktsaou]

btw, @paulfantom for all these fixes I always check if the command will work on busybox. We need it to work on docker (alpine) and embedded devices, so busybox is important.

busybox has -f for ln. So this was ok.

[paulfantom]

From our .travis.yml:

netdata/.travis.yml

Lines 62 to 64 in 5ad6478

	# TODO: cannot run installer without bash
	# - name: "alpine"
	# script: ./.travis/containerized_build.sh alpine