Overwritting GREP_OPTIONS on local script

I found an issue (possibly related to #285 and #262) that `nvm.sh` uses `grep` but doesn't ignore the user's `GREP_OPTIONS` (http://www.gnu.org/savannah-checkouts/gnu/grep/manual/grep.html#Environment-Variables) environment variable, which may change the output expected by `nvm.sh`.

I have overwrite it inside nvm.sh.



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