Merge build logs instead of append it

[wingyplus]

Sometimes server sent build log event since first line and drone-ui
append to the old build log. This approach make a user see a build since
first line again even it already shown. Fix it by merge a new logs to
the old logs by using field "pos" as a array index.

This is an example:

NOTE: I cannot show the full log because it's secret.

[wingyplus]

@bradrydzewski Love to see if you've any suggestions. :)