ignoring .ruby-version
fixes a server compatibility check

* breaks when it encounters executable directories
* now skips checks if server version cannot be determined from the binary
* replaced 'raise' with 'warn' statements
* bumped version to pre3
Merge pull request #356 from calabash/fix/server-compat-scan-breaks-on-executable-dirs

server_version_from_bundle no longer raises error when it fails to detect the server version