Rename all nonfree files (forbidden character)

icons/nonfree_instagram.svg

icons/nonfree_lastfm.svg

icons/nonfree_leboncoin.svg

icons/nonfree_petitbambou.svg

icons/nonfree_shazam.svg

icons/nonfree_skype.svg

icons/nonfree_slack.svg

icons/nonfree_spotify.svg

icons/nonfree_tinder.svg

icons/nonfree_twitter.svg

icons/nonfree_waze.svg

icons/nonfree_whatsapp.svg

icons/nonfree_youtube.svg