Use bash-preexec when it is available

[FranklinYu]

status: blocked by #471.

[postmodern]

This seems like something that needs a test to catch future breaking changes. Could add a test file that checks for bash ([[ -n "$BASH_VERSION" ]] or even [[ ! -n "$ZSH_VERSION" ]]?), source in bash_preexec before chruby/auto.sh, then check if preexec_functions was appended.