Repaired virtualenv activation and deactivation for the fish shell

This pull request repairs https://github.com/yyuu/pyenv-virtualenv/issues/22

Unfortunately there seems to be no way to use command substitution and preserve new lines in the fish shell.  As such, all commands (except the last) must have a semicolon applied explicitly to indicate the end of the command.

I have tested this tiny fix and it works perfectly now (see below):

```
fots@fotsies-ubtrusty-01 ~> env | egrep PYENV
PYENV_ROOT=/home/fots/.pyenv
PYENV_SHELL=fish
fots@fotsies-ubtrusty-01 ~> pyenv activate flaskage
(flaskage)fots@fotsies-ubtrusty-01 ~> env | egrep PYENV
PYENV_ROOT=/home/fots/.pyenv
PYENV_SHELL=fish
PYENV_VERSION=flaskage
(flaskage)fots@fotsies-ubtrusty-01 ~> pyenv deactivate
fots@fotsies-ubtrusty-01 ~> env | egrep PYENV
PYENV_ROOT=/home/fots/.pyenv
PYENV_SHELL=fish
fots@fotsies-ubtrusty-01 ~>
```

Cheers
Fotis