Make update check use diff without comments/whitespace

[Tyil]

This change allows people to change the comments on the .travis.yaml file. The diff ignores whitespace changes as well, so people can use their preferred indentation style as well.

Personally, I'd like to add a .travis.d directory to put the scripts in like the update checker, and allow custom scripts to be dropped in such a directory too. I can add that as well, if you were to agree on this.

[mrueg]

Just to understand what the goal of this change is: Why would you want to change the comments in the script in a downstream version?

[Tyil]

Preferably, I'd like much more customisation and extensibility, but for now, just this should do fine for me. I wanted to change the credits to read "Credits to", as that seems more appropriate to me.

I also don't think checking the filesize is the best way to go about finding out if it differs, there's diff for exactly this purpose.

[samcv]

Any movement on this one? I also agree using diff with ignored whitespace/comments would be a much better way to do it as well.