Try to update Travis image for macOS.

.travis.yml

@@ -41,14 +41,17 @@ matrix:
     env:
     - MATRIX_EVAL="export CC=gcc-7 CXX=g++-7 CONFIG=Release PYTHON=python3 CMAKE_EXTRA='-DLIBIGL_EIGEN_VERSION=3.3.7'"
   - os: osx
+    osx_image: xcode10.2
     compiler: clang
     env:
     - MATRIX_EVAL="export CONFIG=Debug PYTHON=python3 LIBIGL_NUM_THREADS=1"
   - os: osx # same config like above but with -DLIBIGL_USE_STATIC_LIBRARY=OFF to test static and header-only builds
+    osx_image: xcode10.2
     compiler: clang
     env:
     - MATRIX_EVAL="export CONFIG=Debug PYTHON=python3 LIBIGL_NUM_THREADS=1 CMAKE_EXTRA='-DLIBIGL_USE_STATIC_LIBRARY=OFF'"
   - os: osx
+    osx_image: xcode10.2
     compiler: clang
     env:
     - MATRIX_EVAL="export CONFIG=Debug PYTHON=python3 LIBIGL_NUM_THREADS=1 CMAKE_EXTRA='-DLIBIGL_EIGEN_VERSION=3.3.7'""