Don't buffer build output

[anibali]

This is a better fix for the nan version upgrade problem (#124). I submitted an earlier PR which "solved" the issue by fixing the version in packages.json (#125). However, I have since found the root cause of the problem (nodejs/nan#716).

It turns out that with nan 2.8.0, the node-sodium build process prints out so many warnings that child_process.exec reaches max buffer size, causing node-gyp rebuild to be terminated! Since the buffered output is not actually used, this PR solves the issue by replacing exec with spawn. Having nan 2.8.0 as a dependency now works fine.

[anibali]

@paixaop I'm pretty sure that the failing test is unrelated to this PR:

not ok 145 PWHash argon2i should verify the generated hash with same password
  AssertionError: false == true
      at Context.<anonymous> (test/test_crypto_pwhash.js:90:9)

[paixaop]

implemented this change in the master in version 3.0.0. Thanks!