Merge pull request #8 from tcnksm/issues/6

Added Log4r logging.
Refactoring
Add --output option for setting output filename
Add --force option for overwrite dockerfile without prompting
Delete unnessary code
Add man page with gem-man
Refactoring ubuntu pacakge installation
Update the list of available Ruby versions
Merge pull request #10 from oame/update-list-of-ruby

Update the list of available Ruby versions