Fix unmarshal error for recipient on i386

[kostkobv]

Most of the phone numbers (11 numbers) would throw an error during unmarshaling the response from the REST API, due to int would be only 32-bits long on i386 and 32-bits int is < 2,147,483,647 (10 numbers). This small fix would remove this inconvenience

[cooldarkdryplace]

Hey, thanks for your PR.
You are right MSISDN can overflow int32.

The ITU-T recommendation E.164 limits the maximum length of an MSISDN to 15 digits.

This is a breaking change, so we are going to include your fix in new release.

[kostkobv]

@cooldarkdryplace Cheers! See you on the next Golang meetup 😉

[epels]

Hi @kostkobv - thanks a bunch for your much appreciated contribution!

We're planning on releasing the new version next week. Could you please change the target branch to v5_staging? We're making a few more changes, and it's easiest for me to have them all on a single branch before releasing. Thanks in advance!