Added URL checker and corresponding CI job. Fixed all dead links. Removed travis related files.

Signed-off-by: Daniel Scheffler <danschef@gfz-potsdam.de>
4 jobs for v0.4.2 in 50 seconds (queued for 2 minutes and 31 seconds)
latest
Status Name Job ID Coverage
  Test
passed test_pyrsr #52879

00:00:13

77.0%
passed test_styles #52880

00:00:07

passed test_urls #52881

00:00:16

 
  Deploy
passed deploy_pypi #52882

00:00:13