build: Bump required GTK+ version

It's required for margin-start and margin-end.

https://bugzilla.gnome.org/show_bug.cgi?id=712661
background: Use margin-start/end instead of margin-left/right

https://bugzilla.gnome.org/show_bug.cgi?id=712661
color: Use margin-start/end instead of margin-left/right

https://bugzilla.gnome.org/show_bug.cgi?id=712661
network: Use margin-start/end instead of margin-left/right

https://bugzilla.gnome.org/show_bug.cgi?id=712661
datetime: Use margin-start/end instead of margin-left/right

https://bugzilla.gnome.org/show_bug.cgi?id=712661
display: Use margin-start/end instead of margin-left/right

https://bugzilla.gnome.org/show_bug.cgi?id=712661
mouse: Use margin-start/end instead of margin-left/right

https://bugzilla.gnome.org/show_bug.cgi?id=712661
notifications: Use margin-start/end instead of margin-left/right

https://bugzilla.gnome.org/show_bug.cgi?id=712661
power: Use margin-start/end instead of margin-left/right

https://bugzilla.gnome.org/show_bug.cgi?id=712661
printers: Use margin-start/end instead of margin-left/right

https://bugzilla.gnome.org/show_bug.cgi?id=712661
privacy: Use margin-start/end instead of margin-left/right

https://bugzilla.gnome.org/show_bug.cgi?id=712661
region: Use margin-start/end instead of margin-left/right

https://bugzilla.gnome.org/show_bug.cgi?id=712661
common: Use margin-start/end instead of margin-left/right

https://bugzilla.gnome.org/show_bug.cgi?id=712661
info: Use margin-start/end instead of margin-left/right

https://bugzilla.gnome.org/show_bug.cgi?id=712661
keyboard: Use margin-start/end instead of margin-left/right

https://bugzilla.gnome.org/show_bug.cgi?id=712661
online-accounts: Use margin-start/end instead of margin-left/right

https://bugzilla.gnome.org/show_bug.cgi?id=712661
search: Use margin-start/end instead of margin-left/right

https://bugzilla.gnome.org/show_bug.cgi?id=712661
sharing: Use margin-start/end instead of margin-left/right

https://bugzilla.gnome.org/show_bug.cgi?id=712661
universal-access: Use margin-start/end instead of margin-left/right

https://bugzilla.gnome.org/show_bug.cgi?id=712661
user-accounts: Use margin-start/end instead of margin-left/right

https://bugzilla.gnome.org/show_bug.cgi?id=712661