Fix version detection

[Paultagoras]

Problems to solve:

• Reports a wrong client version or update didn’t happen.
• Reports never released kafka connector version (v1.2.10)
• The existing tests haven't caught the problem

Currently we're defaulting to 0.6.3:

public static final String LATEST_KNOWN_VERSION = "0.6.3";

We need to find a more reliable way to determine the version...

[mshustov]

or don't guess and set a well-known indicator like unknown, which would help us to detect problems earlier and fix them

[chernser]

@mshustov we need to know at least aprox. version - so I've added ~ to version constant.
Event if we missed updating it - we will get something ~0.7.2.
Please let me know if this will work.

Thanks!