travis: Upgrade Emacs 26 test matrix element to 26.3
travis: Upgrade Emacs 26 test matrix element to 26.3

Emacs 26.3 was released on 2019-08-28, so probably a good idea to test
against it now.

Signed-off-by: Kristofer Rye <1ed31cfd0b53bc3d1689a6fee6dbfc9507dffd22@gmail.com>
Tested-by: Kristofer Rye <1ed31cfd0b53bc3d1689a6fee6dbfc9507dffd22@gmail.com>

kotct/dot,kotct/dot