[Xastir-dev] Travis CI has Ubuntu-18.04LTS

[Curt Mills]

Looks like Travis CI has Ubuntu-18.04LTS now. We should add builds for
for it. Once working we could remove the Ubuntu-16.04LTS builds. Any
reason not to?

Current Travis CI builds:
    Ubuntu-16.04 light w/ImageMagick
    Ubuntu-16.04 gcc8 full-featured w/GraphicsMagick
    OSX semi-light w/GraphicsMagick
    Ubuntu-16.04 Exercise Configure Options

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Curt, WE7U        http://we7u.wetnet.net        http://www.sarguydigital.com