Bot doesn't realize it's version on released binary (2.1.0-)

[9-9-9-9]

For documentation purpose

2.1.0 and earlier versions, bot able to know it's release version when having source code (pom.xml) file because it reads from it.

But on released versions (zip files) bot can't because the pom.xml file wasn't included into the release

So from 2.2.0 (PR#54), an additional file named current-version.txt was added to resources folder and bot can read it's version from it

Consequence:

• From 2.1.0, the feature to save current version was made, but won't work until 2.2.0
• From 1.7.0, the feature to check for new updates was made, but won't work until 2.2.0