upgrade mason to ensure clang v3.8.0 is available

bootstrap.sh

@@ -10,7 +10,7 @@ todo
 - shrink icu data
 '
 
-MASON_VERSION="3e0cc5a"
+MASON_VERSION="b709931"
 
 function setup_mason() {
     if [[ ! -d ./.mason ]]; then
@@ -31,7 +31,7 @@ function install() {
         mason install $1 $2
         mason link $1 $2
         if [[ ${3:-false} != false ]]; then
-            LA_FILE=$(${MASON_DIR:-~/.mason}/mason prefix $1 $2)/lib/$3.la
+            LA_FILE=$(mason prefix $1 $2)/lib/$3.la
             if [[ -f ${LA_FILE} ]]; then
                perl -i -p -e 's:\Q$ENV{HOME}/build/mapbox/mason\E:$ENV{PWD}:g' ${LA_FILE}
             else