Make sure custom grep options don't get in the way

I saw this error when I first tried to run `stack.sh`:
```
$ ./stack.sh
/home/jason/github/devstack/functions: line 1406: (standard: command not found
/home/jason/github/devstack/lib/databases/mysql: line 147: (standard: command not found
/home/jason/github/devstack/lib/databases/postgresql: line 98: (standard: command not found
/home/jason/github/devstack/lib/database: line 124: (standard: command not found
/home/jason/github/devstack/lib/rpc_backend: line 163: (standard: command not found
WARNING: this script has not been tested on Debian-6.0.7.
[ERROR] ./stack.sh:108 If you wish to run this script anyway run with FORCE=yes
/home/jason/github/devstack/functions: line 98: (standard: command not found
```
It turns out I had the following set in my bash profile:
```
export GREP_OPTIONS="--color=auto -nH"
```
This fix should prevent this issue from occurring when `GREP_OPTIONS` has been customized.