Fix building with upper gtk versions

[EPashkin]

IMHO gtk_3_18 feature better changed to gtk_3_16 for consistency
Also defaults commented as example

[GuillaumeGomez]

Agreed. Thanks @EPashkin.

cc @gkoz

[EPashkin]

Not sure how to change to v3_16: just lower feature or also lower installed version in travis.

[GuillaumeGomez]

Just the feature should be enough.

[EPashkin]

Lowered feature

[EPashkin]

Any 3_18 still won't builds due Gesture

[EPashkin]

Repush to restart builds

[GuillaumeGomez]

@EPashkin: If you want to restart builds, just ask to me or to @gkoz.

[gkoz]

Just removing gtk_3_18 will break appveyor for all other repos.

[EPashkin]

Add it again as gtk_3_18 = ["gtk_3_16"] ?

[EPashkin]

We can also add gtk_latest = ["gtk_3_16"] and update all other repos to it.

[gkoz]

latest isn't too good when looked at from the point of view of the CI build matrix.
I think I'd prefer gtk_3_18 = ["gtk_3_16"] but with a comment pointing out that it's used by CI tools. But then there remains little reason to have gtk_3_16. You just can't win it seems.

[EPashkin]

IMHO main reason for "gtk_3_16" is example of right way of adding versions.

[gkoz]

What are we saying by including this line? It could be interpreted in unpredictable ways.

[EPashkin]

Its example for setting upper version by default

[gkoz]

Anyway, bikeshedding this is too boring. Thanks!