Use CI to build xar

Too often tei-publisher repo is in a state where users can't build the app because of missing or undeclared dependencies. This has been going on since 3.0.0 and was also a problem with 4.0.0RC.

Instead of me harping on about this every time a new features is worked on, why not use the CI service here to simply build the xar and see if that fails.

Edited Sep 21, 2018 by Duncan Paterson

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