MacOS travis build is switched to llvm37

.travis/osx/build.sh

 export PATH=/usr/local/opt/qt5/bin/:$PATH
 mkdir build
 cd build
-cmake -DCMAKE_C_COMPILER=/usr/local/opt/llvm38/bin/clang-3.8 -DCMAKE_CXX_COMPILER=/usr/local/opt/llvm38/bin/clang++-3.8 -DCMAKE_PREFIX_PATH=/usr/local -DCMAKE_BUILD_TYPE=$BUILD_TYPE ..
+cmake -DCMAKE_C_COMPILER=/usr/local/opt/llvm37/bin/clang-3.7 -DCMAKE_CXX_COMPILER=/usr/local/opt/llvm37/bin/clang++-3.7 -DCMAKE_PREFIX_PATH=/usr/local -DCMAKE_BUILD_TYPE=$BUILD_TYPE ..
 make -j4