osx: by default do not install XQuartz

Author: samoht

.travis-ocaml.sh

@@ -44,7 +44,7 @@ UPDATE_GCC_BINUTILS=${UPDATE_GCC_BINUTILS:-"0"}
 UBUNTU_TRUSTY=${UBUNTU_TRUSTY:-"0"}
 
 # Install XQuartz on OSX
-INSTALL_XQUARTZ=${INSTALL_XQUARTZ:-"true"}
+INSTALL_XQUARTZ=${INSTALL_XQUARTZ:-"false"}
 
 install_opam2 () {
     case $TRAVIS_OS_NAME in