Update install script & osxdefaults

- Update osxdefaults to not attempt to write what appears to be a
  default (PromptOnQuit)
- Update osxdefaults to correct path to iterm2 colorscheme
- Update order of operations for install script so that shell change
  occurs after ViM installation as it relies on ViM
- Update print statements to be more clear on install script

Issue: #9