Do not use virtualenv's (de)activate scripts?

[blueyed]

There are several issues with the virtualenv's activate script, especially with its handling of $PATH (pypa/virtualenv#722).

Since it's basically rather trivial for what it does, I think it would be a good idea for pyenv-virtualenv to not use it, but do the setup / restoring itself.

Other issues: pypa/virtualenv#724, pypa/virtualenv#723.

[yyuu]

Avoid using the activate script generated by virtualenv is possible. But I think that keep using vanilla virtualenv would be better for compatibility.