Fixes Bitrise API - Missing Payload

[D3icidal]

Adds --header 'Content-Type: application/json' to the 'trigger_build()' function that seems to now be required by Bitrise
Otherwise receiving a error of: ERROR: null%

Where the json response is hidden behind -d and shows: {"error_msg":"Invalid request: does not contain a 'payload', or it is not a valid JSON request"}

Fixes #65

[D3icidal]

I heard back from Bitrise, this was a unintentional change and they will undo it. But "it would be best to leave it there as it's best to be explicit about the headers anyway but no longer required"

So this can be closed out if needed

[Azbagheri]

@D3icidal thank you for all the context about the change! I agree, this is a safe addition to the API call.