Added GTK 3.4 to older versions list

[dubcanada]

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I added GTK 3.4 to the list. gtktest.rs won't compile on 3.4 due to the newer features being used. But when I commented them out it ran and looked fine. I asked in #rust and there does not seem to be any way to do #ifdef so you'd need to create version specific gtktest files or stub it.

[GuillaumeGomez]

Well, I tested and it seems good to me. I'll wait for @jeremyletang to look at it before merging it.

[jeremyletang]

hi, Sorry for the delay.

You should add #[cfg(GTK_3_4)] ... on the given widget in gtk/mod.rs and gtk/widget/mod.rs to don't build them if you exclude GTK 3.4 and ulterior version. You can see how it's made in the file for other versions for example.

After this change it should be okay to merge.