Setup Travis CI for build AppImage (portable binary package for Linux)

[ghost]

Setup Travis CI for auto-build AppImage (portable package for Linux) for each commit:

0. Create .travis.yml - https://github.com/OpenVSP/OpenVSP/blob/master/.travis.yml
1. https://github.com/appimage/AppImageKit 1.1. https://github.com/probonopd/linuxdeployqt 1.2 https://github.com/linuxdeploy
2. https://github.com/probonopd/uploadtool
3. Enable Travis CI pushing to GitHub releases for this repo.

For example, take a look on LeoCAD .travis.yml implementation:

• https://github.com/leozide/leocad/blob/master/.travis.yml

And here is what resulted builds would look like:

• https://github.com/leozide/leocad/releases/tag/continuous

For any support:

• https://docs.appimage.org/user-guide/faq.html#question-where-do-i-get-support

[guillaumechereau]

Closing: I switch from Travis to GitHub CI system.