Add checking of lua sources to travis build

[HHHartmann]

• This PR is for the dev branch rather than for master.
• This PR is compliant with the other contributing guidelines as well (if not, please describe why).
• I have thoroughly tested my contribution.

There are several *.lua files that are not compilable.
This PR adds checking lua files in travis builds.
Travis build will fail if there is a lua file in lua_modules or lua_samples that cannot be compiled.

[HHHartmann]

So found two more build errors in Lua code
Ready for integration now. (from my POV only of corse)

[TerryE]

@HHHartmann Gregor normally what I do when I am working on a patch is to rebaseline against (the current) dev before pushing to my GitHub fork and issuing the PR. In my case because my patches are often big and take time, dev has usually moved on since I created my branch so this is even more needed, but we recommend this practice. Maybe for your next commit, eh? 😊

[HHHartmann]

@TerryE Terry I do not quite understand what you ask me to do (or even not to do). Shouldn't I have merged the remote-tracking branch? I am not yet very familiar with git so I would be more than happy to be enlightened a bit more.
But most of all let me thank you for the great LFS patch. It is great to have telnet and ftp running and still have 40k of RAM left.

[TerryE]

@HHHartmann Gregor, it is far more important that you feel comfortable contributing which is why Marcel and I were happy to merge your PR with multiple commits. The rebasing stuff is covered here in the online Git book, but to be honest I get confused with the advanced features of git at times! Sometime I'll get a round to a simple 1-2-3 guide.

[HHHartmann]

@TerryE Terry, thanks for the link. I will read it and find out how to do it. But still I wonder what I should have done? Should it have been only one commit? Puzzled and curious.

[TerryE]

But still I wonder what I should have done

For a change of this size and scope I would had one or two commits, and if two then I would done the *.lua changes as one commit.

[HHHartmann]

Ah ok.
I had to do separate commits here because I could see no way to test beforehand as the travis build only gets triggered there. But I suppose I could have squashed them afterwards? Or reduce them by first thinking, then committing.
Thanks four your guidance anyways.