Be more explicit about `.python-version` file

When I came back to this after a few months - despite setting it up properly previously - I was a bit confused since I'm so used to setting an actual Python version in `.python-version` files. Explain exactly how the file should look.

Use `set -gx` instead of `setenv` for fish shell.

Fix test "activate if the first virtualenv is a virtualenv": it was asserting
the output was of bash form, but not specifying that in the test — it now does.

Fixes https://github.com/pyenv/pyenv-virtualenv/issues/216.

Fix typo

Fix the installation command of README.md

Use `-m venv` instead of `pyvenv` executable (fixes #184)

Use `source` in favor of `.` (fixes #175)

* https://github.com/yyuu/pyenv/pull/615
* https://github.com/fish-shell/fish-shell/issues/310

Source conda package activate/deactivat scripts if exist

Simplified messaging levels

Suppress activate/deactivate messages by default

change minor typo in bin/pyenv-sh-activate

Evaluate ${PATH} when outputted code is eval'd.

`pyenv virtualenvs` lists two entries for each virtualenv

It was previously evaluated when 'pyenv virtualenv-init' is run.