Let FileStream support bytesRead.

[ahuigo]

Let FileStream inherits fs.ReadStream to support bytesRead property like this:
#173

busboy.on('file', function(fieldname, file, filename, encoding, mimetype) {
          const fs = require('fs')
          const ws = fs.createWriteStream('busboy.txt')
          const ps = file.pipe(ws)
          ps.on('finish', ()=>{
            console.log(file.bytesRead); // support bytesRead here
            console.log(ws.bytesWritten) 
            console.log(ps.bytesWritten) 
          })
    });

[ahuigo]

Why not remove old node versions?

  matrix:
  - TRAVIS_NODE_VERSION="0.10"
  - TRAVIS_NODE_VERSION="0.12"