This fixes displaying the note, in case "init" has been run manually
and/or the init is not in the expected place.
It removes the unnecessary calls to "grep".
pyenv-virtualenv-init will look at `$PYENV_SHELL` now, too.
Tests have been adjusted/fixed.
When the current pyenv environment is not a virtualenv, this line
exits with a return code of 1. This is annoying for those of us
who report the last return code in our own PROMPT_COMMAND, so
ignore the output of `pyenv activate` if we're not currently using
a virtualenv.
The situation is when in a directory containing python binary,
`pyenv prefix' will return empty (caused by `pyenv which' returning
"./python"). This change adds quotes for `test', otherwise `test' will
warn about missing argument.
