Search instead of direct compare when checking install script

Some modules are using variations on "node-gyp rebuild" in their install scripts (e.g. [ws](https://github.com/einaros/ws/blob/76a1ae3dde7c190f46f54a85c0fcf303de3c4351/package.json#L15) as of this writing), so it would be nice to instead do a substring search instead of an exact comparison. Making this change will prevent the need to manually set 'gypfile' to true for such packages.