Add Travis test with empty NODE_PATH

[gaearon]

Since Travis tests run with NODE_PATH specified, it doesn't catch issues in code that does something wrong on empty NODE_PATH. We should add end-to-end tests for starting, building, and running tests with empty NODE_PATH.

[BrodaNoel]

This should be tackled in this file, right?

[gaearon]

Yeah.

[stale]

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