add current release version number in shaarli_version.txt
 * release process: please update this file when releasing a new version on github
update check: check against last version available on github.com/shaarli/Shaarli
 * fixes https://github.com/shaarli/Shaarli/issues/5
Merge pull request #31 from nodiscc/version-check

Version check: check against latest version on github