title: CI for Qt app: build with different Qt versions
body: <p>I use Travis-CI for continuous integration for my simple Qt app. My .travis.yml file looks like this (based on <a href="https://gist.github.com/jreese/6207161" rel="noreferrer">this gist</a>):  </p>
  
  <pre><code>language: cpp
  
  before_install:
    - sudo add-apt-repository --yes ppa:ubuntu-sdk-team/ppa
    - sudo apt-get update -qq
    - sudo apt-get install -qq g++ qt4-qmake libqt4-dev qt5-qmake qtbase5-dev
  
  script: 
    - qmake -qt=qt4 -v
    - qmake -qt=qt4
    - make
    - make -k check
    - make clean
    - qmake -qt=qt5 -v
    - qmake -qt=qt5
    - make
    - make -k check
  </code></pre>
  
  <p>This configuration allows me to build my app (and run tests) with default Qt libraries in Ubuntu (Qt 4.8.1 and Qt 5.0.2).  </p>
  
  <p>Is there any way to build app with other Qt versions (4.7.x, 4.8.x, 5.1.x and so on)?</p>