Fix for 'make' failure when process.env.JOBS is '0'

If you use the environment variable `JOBS` which are set '0' for other purpose,
`node-gpy rebuild` will fail to execute `make`.
Because the `-j` option of `make` require a non-zero positive integer.

Here is a simple repro:

```
$ JOBS=0 npm install nightmare
...
> weak@0.3.3 install /home/kui/tmp/nm/node_modules/nightmare/node_modules/phantom/node_modules/dnode/node_modules/weak
> node-gyp rebuild

make: the `-j' option requires a positive integral argument
Usage: make [options] [target] ...
Options:
  -b, -m                      Ignored for compatibility.
  -B, --always-make           Unconditionally make all targets.
  -C DIRECTORY, --directory=DIRECTORY
                              Change to DIRECTORY before doing anything.
  -d                          Print lots of debugging information.
..
```
Merge pull request #511 from kui/fix-zero-env-jobs

Fix for 'make' failure when process.env.JOBS is '0'