Version numbering of the shared libraries in debian packages

Version numbering of the shared libraries in debian packages

This PR changes naming of the shared libraries. Since ABI breaking is done per minor version up, minor version number needs to be appear to the shared libraries.

For example:
* `libdart-core4.2` package installs `libdart-core.so.4.2`
* `libdart-core4-dev` package installs `libdart-core.so.4` symlink

This PR should fixes #281.
The -dev packages usually host the symlinks (.so) that points to real libraries (.so.4.2). Any reason to use here a .so.4?