Swift 4.2 support.

[glm4]

Migrate swift version to 4.2 in all targets and dependencies.

[anphanngocvn]

@glm4 This latest upgrade doesn't stable. The onReceive callback works sometime (the same issue to version 0.2.4). However if I revert to 0.2.3, then it works very well.

[amakhonin]

@glm4 This latest upgrade doesn't stable. The onReceive callback works sometime (the same issue to version 0.2.4). However if I revert to 0.2.3, then it works very well.

this happens because if let item = idJSON.first { channelIdentifier = item.value as? String } in JSONSerializer, guess we need another fork..

[glm4]

@glm4 This latest upgrade doesn't stable. The onReceive callback works sometime (the same issue to version 0.2.4). However if I revert to 0.2.3, then it works very well.

I guess that's not in the scope of this PR.