Continuous builds on Travis CI

[probonopd]

This PR, when merged, will compile EKO on Travis CI upon each git push, and upload an AppImage to a temporary download URL on transfer.sh (available for 14 days). The download URL is toward the end of each Travis CI build log of each build.

For this to work, you need to enable Travis CI for your repository as described here.

Providing an AppImage would have, among others, these advantages:

• Works for most Linux distributions (including Ubuntu, Fedora, openSUSE, CentOS, elementaryOS, Linux Mint, and others)
• One app = one file = super simple for users: just download one AppImage file, make it executable, and run
• No unpacking or installation necessary
• No root needed
• No system libraries changed
• Just one format for all major distributions
• Works out of the box, no installation of runtimes needed
• Optional(!) desktop integration with appimaged
• Binary delta updates, e.g., for continuous builds (only download the binary diff) using AppImageUpdate
• Can GPG2-sign your AppImages (inside the file)

Here is an overview of projects that are already distributing upstream-provided, official AppImages.

An example AppImage can be downloaded from here: https://transfer.sh/KtSsl/eko-git.e89d36e-x86-64.appimage (available for 14 days)

[probonopd]

Thanks for merging. For this to work, you need to enable Travis CI for your repository as described here.

[psemiletov]

Works like a charm! Thanks a lot! Travis CI is a some sort of magic :)