Refine travis config to only install qt4 or qt5 per build instead of both

.travis.yml

 language: cpp
 env:
-  - QT="-DWITH_QT4=0"
-  - QT="-DWITH_QT4=1"
+  - QT4=1
+  - QT4=0
 os:
   - linux
   - osx
 compiler:
   - gcc
   - clang
-script: mkdir build && cd build && cmake .. $QT -DWITH_SERVER=1 && make
+script: mkdir build && cd build && cmake .. -DWITH_QT4=$QT4 -DWITH_SERVER=1 && make
 install: ./travis-dependencies.sh
 cache: apt

travis-dependencies.sh

@@ -2,10 +2,19 @@
 
 if [[ $TRAVIS_OS_NAME == "osx" ]] ; then
   brew update
+  if (( QT4 )); then
+    brew install qt protobuf libgcrypt
+  else
     brew install qt qt5 protobuf libgcrypt
+  fi
 else
+  if (( QT4 )); then
+    sudo apt-get update -qq
+    sudo apt-get install -y qtmobility-dev libprotobuf-dev protobuf-compiler libqt4-dev
+  else
     sudo add-apt-repository --yes ppa:ubuntu-sdk-team/ppa
     sudo apt-get update -qq
-  sudo apt-get install -y qtmobility-dev libprotobuf-dev protobuf-compiler libqt4-dev\
-   qtbase5-dev qtdeclarative5-dev libqt5webkit5-dev libsqlite3-dev qt5-default qttools5-dev-tools
+    sudo apt-get install -y libprotobuf-dev protobuf-compiler qtbase5-dev\
+      qtdeclarative5-dev libqt5webkit5-dev libsqlite3-dev qt5-default qttools5-dev-tools
+  fi
 fi