Optimize pyenv-which: implement remove_from_path in Bash

This greatly improves the performance of `pyenv virtualenvwrapper_lazy`,
which happens to call pyenv-which a lot.

For profiling I've initially used:

    % zmodload zsh/zprof
    % eval "$(pyenv init -)"
    % pyenv virtualenvwrapper_lazy

    Before:
    % zprof|grep -E '(pyenv|virtualenv)'
     1)    1         754,07   754,07   58,95%    751,50   751,50   58,75%  pyenv
    21)    1           2,57     2,57    0,20%      2,57     2,57    0,20%  virtualenvwrapper_setup_lazy_loader
     1)    1         754,07   754,07   58,95%    751,50   751,50   58,75%  pyenv
           1/1         2,57     2,57    0,20%      2,57     2,57             virtualenvwrapper_setup_lazy_loader [21]
           1/1         2,57     2,57    0,20%      2,57     2,57             pyenv [1]
    21)    1           2,57     2,57    0,20%      2,57     2,57    0,20%  virtualenvwrapper_setup_lazy_loader

    After:
    % zprof|grep -E '(pyenv|virtualenv)'
     1)    1         383,30   383,30   27,97%    380,88   380,88   27,79%  pyenv
    31)    1           2,42     2,42    0,18%      2,42     2,42    0,18%  virtualenvwrapper_setup_lazy_loader
     1)    1         383,30   383,30   27,97%    380,88   380,88   27,79%  pyenv
           1/1         2,42     2,42    0,18%      2,42     2,42             virtualenvwrapper_setup_lazy_loader [31]
           1/1         2,42     2,42    0,18%      2,42     2,42             pyenv [1]
    31)    1           2,42     2,42    0,18%      2,42     2,42    0,18%  virtualenvwrapper_setup_lazy_loader

Fixes https://github.com/yyuu/pyenv-virtualenvwrapper/issues/13
Merge tests and remove_from_path from rbenv

This adds the missing parts from the upstream rbenv merge:
https://github.com/sstephenson/rbenv/commit/f4652fbbf0bbc3d07a36e8781b2be54ec1f75518

- https://github.com/sstephenson/rbenv/commit/e4cbf04592cfcd9d244fb83284e14524c8ba0377
- https://github.com/sstephenson/rbenv/commit/3ee395f9b576e4111650cef67328f4ffe3687b5a
Add test for PYENV_VERSION=3.4:2.7
Merge rbenv's "Isolate rbenv-which tests …" (c69d9a11)

This is the remaining part of
https://github.com/sstephenson/rbenv/commit/c69d9a1128283f20ad883178e3649d7ed92be663.

    commit c69d9a1128283f20ad883178e3649d7ed92be663
    Author: Mislav Marohnić <mislav.marohnic@gmail.com>
    Date:   Mon Oct 13 12:39:47 2014 +0200

        Isolate rbenv-which tests from any `.ruby-version` file on the system

        Having a `.ruby-version` file in any of the parent directories of the
        local clone of rbenv could cause the test suite to fail because it
        wasn't expecting a local version to be set.
Merge pull request #129 from blueyed/optimize-pyenv-which

Optimize pyenv-which: implement remove_from_path in Bash