Replace event-stream by through2

[louis-bompart]

event-stream, one of the dependencies of the package, have been backdoored (see dominictarr/event-stream#115).
To avoid potential issues, I would recommend using a smaller package that does just what's needed, like
through or, as proposed in this PR through2. Both are imho valid choices, I just chose through2 because it has a higher download count on npm and its latest update is more recent.

[tatsuyafw]

@louis-bompart

Thanks for your PR!
But It seems that your PR causes errors as follows 😢

events.js:183
      throw er; // Unhandled 'error' event
      ^

TypeError: Invalid non-string/buffer chunk
    at validChunk (/Users/hoshino/src/github.com/louis-bompart/gulp-nightwatch/node_modules/readable-stream/lib/_stream_writable.js:304:10)
    at DestroyableTransform.Writable.write (/Users/hoshino/src/github.com/louis-bompart/gulp-nightwatch/node_modules/readable-stream/lib/_stream_writable.js:332:62)
    at write (/Users/hoshino/src/github.com/louis-bompart/gulp-nightwatch/node_modules/vinyl-fs/node_modules/readable-stream/lib/_stream_readable.js:623:24)
    at flow (/Users/hoshino/src/github.com/louis-bompart/gulp-nightwatch/node_modules/vinyl-fs/node_modules/readable-stream/lib/_stream_readable.js:632:7)
    at DestroyableTransform.pipeOnReadable (/Users/hoshino/src/github.com/louis-bompart/gulp-nightwatch/node_modules/vinyl-fs/node_modules/readable-stream/lib/_stream_readable.js:664:5)
    at emitNone (events.js:106:13)
    at DestroyableTransform.emit (events.js:208:7)
    at emitReadable_ (/Users/hoshino/src/github.com/louis-bompart/gulp-nightwatch/node_modules/vinyl-fs/node_modules/readable-stream/lib/_stream_readable.js:448:10)
    at emitReadable (/Users/hoshino/src/github.com/louis-bompart/gulp-nightwatch/node_modules/vinyl-fs/node_modules/readable-stream/lib/_stream_readable.js:444:5)
    at readableAddChunk (/Users/hoshino/src/github.com/louis-bompart/gulp-nightwatch/node_modules/vinyl-fs/node_modules/readable-stream/lib/_stream_readable.js:187:9)

[louis-bompart]

Hey sorry for the delay, I misread some stuff in through2, so yeah, through should work just fine.
I updated my PR accordingly, lemme know if the issue still persists.

[tatsuyafw]

Hey sorry for the delay, I misread some stuff in through2, so yeah, through should work just fine.
I updated my PR accordingly, lemme know if the issue still persists.

@louis-bompart

Thanks! Works fine!

[tatsuyafw]

through2 -> through ?

In addition, I think that event-stream package is no longer needed.

[louis-bompart]

True! I'll did that a bit in a hurry, sorry for that.
I'll update the package.json

[tatsuyafw]

LGTM!

[tatsuyafw]

@louis-bompart Thanks!