Add Travis CI builds

[probonopd]

This PR, when merged, will build GammaRay on Travis CI each git push, and upload an AppImage to travis.ci (the download URL is toward the end of the Travis CI build log).

For this to work, please merge this and follow https://travis-ci.org/getting_started

Providing a continuous 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 testers/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
• Optional binary delta updates, e.g., for continuous builds (only download the binary diff) using AppImageUpdate
• Can optionally GPG2-sign AppImages (inside the file)

A growing number of Qt based applications are already being made available in AppImage format for testing by their respective authors, including Kdevelop, MuseScore, Krita, Subsurface, digiKam, and others.

[probonopd]

Ping

[winterz]

I'm for this. I've had a travis build for GammaRay on by todo list for a long time.
my only objection is I'd also like to have a build for macos -- I suppose someone can add that later.
also can we update to use qt5.11.2?

I don't know much about AppImage so I can't comment on that part of the commit.

[probonopd]

Ping

[winterz]

merged . let's see what happens
recently I haven't had much luck with travis. not sure if we're configured properly.

[probonopd]

Yes, Travis CI has been behaving strangely lately; I have moved most of my builds to GitHub Actions since.