Add only the current project's bin directory to $PATH

[croaky]

From https://github.com/sstephenson/rbenv/wiki/Understanding-binstubs#adding-project-specific-binstubs-to-path

export PATH="$PWD/bin:$PATH"
hash -r 2>/dev/null || true

[croaky]

This is a similar attempt to standardize our binstub usage as #166. Our research card is at https://trello.com/card/use-bin-for-binstubs-directory/50f06fb01e671a2e2200ad9a/107

[croaky]

I should note that I don't understand what hash -r 2>/dev/null || true is doing.

[pbrisbin]

hash -r is required when changing PATH because the shell caches program lookup. The redirect and or-true is probably just so that the function operates silently and returns successfully regardless of what hash has to say about it.

[croaky]

Trying an approach @joshuaclayton mentioned:

#191