Add 'bin/ensure-up-to-date' script and update Shippable CI

[jmini]

Fix for #80

[jmini]

Build of aa320fe now looks like the way I expect.

Shippable — Run 345 status is FAILED.

With this in the logs:

UNCOMMITTED CHANGES ERROR
There are uncommitted changes in working tree after execution of 'bin/ensure-up-to-date'
On branch issue80_ensure_samples_up-to-date
Your branch is up to date with 'origin/issue80_ensure_samples_up-to-date'.

Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)

	modified:   samples/client/petstore/ruby/docs/FakeApi.md

no changes added to commit (use "git add" and/or "git commit -a")
Please run 'bin/ensure-up-to-date' locally and commit changes

Now I will update the outdated sample and see if the build is green.

[jmini]

Build of 06e1a14 looks good to me:

Shippable — Run 350 status is SUCCESS.

And in the Logs:

./bin/ensure-up-to-date
# START SCRIPT: ./bin/ensure-up-to-date
IMPORTANT: this script ...
....
Git working tree is clean

[jmini]

Build error of f86bb02

Shippable — Run 357 status is FAILED.

is correct. Logs:

UNCOMMITTED CHANGES ERROR
There are uncommitted changes in working tree after execution of 'bin/ensure-up-to-date'
HEAD detached at FETCH_HEAD
Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)

	modified:   samples/client/petstore/kotlin/README.md
	modified:   samples/server/petstore/kotlin-server/ktor/src/main/kotlin/org/openapitools/server/models/Order.kt
	modified:   samples/server/petstore/kotlin-server/ktor/src/main/kotlin/org/openapitools/server/models/Pet.kt

no changes added to commit (use "git add" and/or "git commit -a")
Please run 'bin/ensure-up-to-date' locally and commit changes

[jimschubert]

@jmini it looks like you're doing some iterative verifications on this branch. Can you create/add a WIP label if this isn't ready to merge?

[jmini]

It is ready to merge, and the CI is green.

We can add more generators to the list of the generators we want to have covered by the check with separated PRs.

I would like to merge this now, if you agree...

---

I use "WIP" prefix in the title of PR when it is requested. In this case, might have noticed that the Shippable CI was only red of a couple of minutes at each iteration... I have verified the state and I have pushed the "solving commit" (the one that update the samples/ folder) right after the end of the failing job.

[jimschubert]

Sounds good! I'll go ahead and merge. I wasn't sure if you were adding all generators, but I agree doing it on separate PRs.