Add gradualizer to ci

[aleklisi]

This PR adds Gradualizer to CI build.
I'd like to try it here before trying it on MongooseIM.

[NelsonVides]

I think we'd really need here to have a

{gradualizer_opts, [{print_file, module}]}.

Otherwise, as you can see on the current Travis build, it raises error but it doesn't say in which file, which is pretty much confusing xD

[NelsonVides]

Uff, no, this one, like all the ones in this file, are I think a by-product of your computer maybe connecting to github through https and not through ssh. I don't think we need to change that.

Also, there's no new field for Gradualizer in your rebar.lock, how comes? 🤔

[NelsonVides]

I think that we'd need this one in Travis's allow_failures block, like Gradualizer's team themselves have here: https://github.com/josefs/Gradualizer/blob/97e8a9363fb21538c4a7223278ab59499ca2be19/.travis.yml#L9

I think that we could change quite a bit this travis.yml actually, to something like

sudo: false

language: erlang
otp_release:
  - 22.1
  - 21.3

branches:
  only:
    - master
    - /BUILD.*/

addons:
  apt:
    packages:
      - libexpat1-dev

install: rebar3 compile
script:
  - rebar3 xref
  - rebar3 dialyzer

matrix:
  allow_failures:
    - script: rebar3 gradualizer

cache:
  directories:
    - $HOME/.cache/rebar3

[NelsonVides]

Also, I'd definitely go for the original josefs repo instead of your fork, I don't know how much trouble you might have at keeping your up-to-date. They just pushed a fix for that OTP22 problem you were having 🙂