Add build-essential to travis cfg

This can't be causing the failure can it? It can't find QtMultimedia..

.travis.yml

@@ -5,5 +5,5 @@ compiler:
 script: mkdir build && cd build && cmake .. && make
 before_install:
   - sudo apt-get update -qq
-  - sudo apt-get install -y qtmobility-dev libprotobuf-dev protobuf-compiler libqt4-dev
+  - sudo apt-get install -y qtmobility-dev libprotobuf-dev protobuf-compiler libqt4-dev build-essential
 cache: apt