Unset `GREP_OPTIONS` to avoid unexpected behavior on usage of grep.

The `GREP_OPTIONS` is an extension of GNU grep, and will not have any effects for BSD grep. If the options are conflicting with the usage of grep in `rbenv`, grep will not perform pattern matching anymore.

```
$ grep --version
grep (GNU grep) 2.15
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by Mike Haertel and others, see <http://git.sv.gnu.org/cgit/grep.git/tree/AUTHORS>.
$ export GREP_OPTIONS="-E"
$ rbenv install -l
Available versions:
grep: conflicting matchers specified
```

refs yyuu/pyenv#101