Fix grep matcher option conflict in 'pyenv install -l' command.

I have set `GREP_OPTIONS="-E"` in my `~/.bashrc` as I normally want grep to work regular expression enabled. 
But in this case, `pyenv install -l` command does not work as followings:

````
$ pyenv install -l
Available versions:
grep: conflicting matchers specified
$
````

This is because grep option -F(Fixed strings matcher) is used in the script and this matcher option conflicts with the other option like -E(Extended regexp matcher), -G(Basic regexp matcher), -P(Perl regexp matcher).

So I prefixed `env --unset=GREP_OPTIONS` to the `grep` in the script, and this worked for me.

````
$ pyenv install -l
Available versions:
  2.4
  2.4.1
  2.4.2
  ...
````

It is much appreciated if you would take a look at this pull request.

Thanks.