Fix requirements-extra.txt Trains package to release version

[bmartinn]

@Borda , I noticed the requirements-extra.txt points to an RC version of Trains.
Since if in a requirements.txt the minimum version is an RC version, pip will always pull the latest pre-release version. I don't think this is what we want, especially with the CI :)
I fixed requirements-extra.txt to point to the latest released version.

Apologies for the mess ...

EDIT:
For the sake of completeness, I also added a reference to Trains in the README visualization section

[Borda]

@bmartinn Thx, it seems that you worked with some older version...
could you please rebase on on actual master?

[bmartinn]

@Borda, I already just (2 hours ago) merged the latest Master branch into my branch and pushed the PR.
GitHub somehow shows previous commits that are unrelated...
Am I missing something ?

[Borda]

that is the difference between merge and rebase... merge just add the missing commits to your branch... on the other hand, rebase takes mater and place your commit on the top so the diff shows just the newly added commits, pls see https://medium.com/datadriveninvestor/git-rebase-vs-merge-cc5199edd77c

[bmartinn]

@Borda, my deepest apologizes, I can totally see my mistake. Let me see if I can fix it...

EDIT:
A clean PR here

[Borda]

@bmartinn no need for an apology! I meant it just as friendly advice and we thank you for you contribution!

[Borda]

another FRIENDLY advice, there is no need to creating new Prs, after the rebase you just git push --force to the actual branch and it updates the PR :]

[bmartinn]

Actually the PR closed automatically the minute I forced pushed the upstream/master head (I guess since the original PR commit id was deleted ...)

[Borda]

yeah, true it closes if the content is identical, so it is better to push it after rebase... it is also possible to reopen it but never mind, let's continue :]