leanpub fixed new images

- The image images/main/terminal_profile_preferences.png has a really
low resolution. We have fixed it for you. You don't need to take any
action. This image is used in these files: main/ruby_tools.md

- The image images/main/terminal_title_and_command.png has a really low
resolution. We have fixed it for you. You don't need to take any
action. This image is used in these files: main/ruby_tools.md
minor fixes for rvm chapter