Set Qt5_DIR in Travis to allow cmake to find Qt5 libraries.

.travis-build.sh

@@ -8,6 +8,7 @@ if [ "$TRAVIS_OS_NAME" = linux -o -z "$TRAVIS_OS_NAME" ]; then
     cmake -DUSE_QT5=OFF .. 
     make -j4
 elif [ "$TRAVIS_OS_NAME" = osx ]; then
+    export Qt5_DIR=$(brew --prefix)/opt/qt5
     mkdir build && cd build
     cmake .. -GXcode
     xcodebuild