Remove -Werror=aggregate-return from travis flags

[techee]

While in general one should avoid returning big structs for performance
reasons, it's no big problem for small structs and it can lead to more
readable code.

Having this check on for everything seems to be too strict - if someone
is returning big structs like crazy, someone will probably notice during
the review. Otherwise it's no problem.

[elextr]

Agree and the same should happen with Geany itself, despite @b4n (IMHO old fashioned ugly and unneccessary) request.

[b4n]

There are other problems why Travis doesn't pass because of lineoperations (cppcheck complaining about useless checks), but this part LGTM.

I still think I prefer not to see too much aggregate returns in Geany, but in GP I don't mind. And for Geany itself I need to reconsider today's implications, maybe it's indeed kind of pointless.