Install dependencies on TravisCI to enable more features

Install Ubuntu development packages for PulseAudio, PortAudio, ALSA and
JACK to enable the building of most Linux backends on TravisCI.

Intall Ubuntu development packages for Qt5 to enable `alsoft-config`.

.travis.yml

@@ -2,5 +2,19 @@ os:
   - linux
   - osx
 dist: trusty
+sudo: required
 language: c
+install:
+  - >
+    if [[ "${TRAVIS_OS_NAME}" == "linux" ]]; then
+      # Install pulseaudio, portaudio, ALSA, JACK dependencies for
+      # corresponding backends.
+      # Install Qt5 dependency for alsoft-config.
+      sudo apt-get install -qq \
+        libpulse-dev \
+        portaudio19-dev \
+        libasound2-dev \
+        libjack-dev \
+        qtbase5-dev
+    fi
 script: cmake . && make -j2