contrib/jenkins: run 'rebar3 fmt --check'
The pull request "v0.0.2" had a patch that formatted everything with
erlfmt. Let's run it in CI to keep the style consistent.
Related: https://github.com/onomondo/onomondo-eim/pull/10/commits/8bafdb3839d868d6bc18499617761d9acd73d2ad
Change-Id: I271c90dace6e7b6f052b38054815e6dfcbfa60f5
