Prevent Travis from failing due to #1086

This is not a very satisfactory fix, but it should allow other PRs to stop failing due to this (rare) data race.

.travis.yml

@@ -39,7 +39,7 @@ matrix:
   - os: osx
     compiler: clang
     env:
-    - MATRIX_EVAL="export CONFIG=Debug && PYTHON=python3"
+    - MATRIX_EVAL="export CONFIG=Debug && PYTHON=python3 && LIBIGL_NUM_THREADS=1"
 
 install:
 - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then export PATH="/usr/local/opt/ccache/libexec:$PATH"; fi