Fix buffer deprecation.

[exoego]

This requires to drop legacy Node.js priort to 5, since Buffer factory
methods are added in Node.js 5.10.

[exoego]

Test fails due to
https://travis-ci.org/crypto-browserify/browserify-sign/jobs/584394970#L1026-L1028

/home/travis/build/crypto-browserify/browserify-sign/node_modules/bn.js/lib/bn.js:3190
   a.umod(this.m)._forceRed(this)._move(a);
                                  ^

[exoego]

Now test failed due to https://travis-ci.org/crypto-browserify/browserify-sign/jobs/639792703#L592

# dsa with 1024 keys
internal/crypto/sig.js:26
  this._handle.init(algorithm);
               ^
Error: Unknown message digest
    at new Sign (internal/crypto/sig.js:26:16)
    at Object.createSign (crypto.js:137:10)
    at Test.<anonymous> (/home/travis/build/crypto-browserify/browserify-sign/test/index.js:66:25)
    at Test.bound [as _cb] (/home/travis/build/crypto-browserify/browserify-sign/node_modules/tape/lib/test.js:80:32)
    at Test.run (/home/travis/build/crypto-browserify/browserify-sign/node_modules/tape/lib/test.js:96:10)
    at Test.bound [as run] (/home/travis/build/crypto-browserify/browserify-sign/node_modules/tape/lib/test.js:80:32)
    at Immediate.next (/home/travis/build/crypto-browserify/browserify-sign/node_modules/tape/lib/results.js:81:19)
    at runCallback (timers.js:705:18)
    at tryOnImmediate (timers.js:676:5)
    at processImmediate (timers.js:658:5)

[calvinmetcalf]

v4.1.0 handles this