Update .travis.yml

Use clang and GCC to build and test mixxx. It is also a recommended practice for Travis to update the virtual machine on every run.
I also changed the scons options to the ones we use on the jenkins build server. It mainly concerns some plugins that are disabled in the default settings.

.travis.yml

@@ -1,14 +1,17 @@
 language: c++
-# command to install dependencies
+# to run compilation/tests with gcc and clang
+compiler:
+    - gcc
+    - clang
 before_install:
+  # update virtual machine
+  - sudo apt-get update -qq
   # Dependencies from  <http://mixxx.org/wiki/doku.php/compiling_on_linux>
   - sudo apt-get install git scons libqt4-dev libqt4-sql-sqlite libportmidi-dev libshout3-dev libtag1-dev libprotobuf-dev protobuf-compiler libvamp-hostsdk3 vamp-plugin-sdk libusb-1.0-0-dev libfftw3-dev libmad0-dev portaudio19-dev libchromaprint-dev librubberband-dev libsqlite3-dev libsndfile1-dev libflac-dev libid3tag0-dev
-  # Virtual X
+  # Virtual X, needed for analyser waveform tests
   - "export DISPLAY=:99.0"
   - "sh -e /etc/init.d/xvfb start"
 install:
-  - scons
-  - scons test=1
+  - scons test=1 mad=1 opus=1 localecompare=1
 script:
-  # `scons test` can't connect to X server for some reason
   - ./mixxx-test