move setting the `~/.bin` in the PATH

Moves it to the start of the bash dotfiles evals.  No behavior
change - just reorgs.
add the `update_terminal_cwd` to the PROMPT_COMMAND

This allows new terminals tabs/windows to open in the last working
dir.  rb sets a custom PROMPT_COMMAND and this jacks with OSX
terminal preferences for this behavior so it needs to be set
manually.