Unset `GREP_OPTIONS` to avoid unwanted 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 option is conflicting with the usage of
grep in `rbenv`, `rbenv` might not work expectedly.

```
$ export GREP_OPTIONS="-E"
$ rbenv install -l
Available versions:
grep: conflicting matchers specified
```