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.

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