Compile on Travis CI

[probonopd]

This PR, when merged, will generate and upload an AppImage for each git push. The download links are in the Travis CI build log.

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.

[probonopd]

You also need to enable Travis CI as described in https://travis-ci.org/getting_started.

Then, on each git push, a new build will appear on https://travis-ci.org/LongSoft/UEFITool/builds.