* Fixes in CI (#406)
* Create .gitattributes
* Update .travis.yml
* Update .travis.yml
* travis fix
* only errors are printed in travis log
* spreadsheet errors fixed: duplicate ID in non-visual properties and lack of third axis in an example
* set-strict example fix
* build-examples.sh exits with error status in the case of build error
* redundant command deleted from script
* unneeded image relationships removed
* pptx validation errors fixes
* all output to travis
Co-authored-by: Gunnsteinn Hall <gunnsteinn.hall@gmail.com>