Version check: check against latest version on github

Fixes https://github.com/shaarli/Shaarli/issues/5

It replaces the check to http://sebsauvage.net/files/shaarli_version.txt with https://raw.githubusercontent.com/shaarli/Shaarli/master/shaarli_version.txt (file in the shaarli/Shaarli repository, has to be updated on every new release)