Silently disable the linter if its command does not exist

More details about this bug: AtomLinter/linter-shellcheck#3 

I resolve this bug by silently disable the linter if its command does not exist. I think there should be a better way, for example, pop up a dialog to let the user choose disable the linter, or specify the linter command path manually.

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***The current commit (5c26e8e) resolve the performance issue. Please review.***

Yes, you are right. I don't know such a difference between `exec` and `spawn`, sorry about that.