add support for OSX

when substituting "in place" with the -i flag using the sed in Mac OS X,
you are required to supply an extension for backing up the file.

Author: kadaliao

export.sh

@@ -1,4 +1,9 @@
+_os="`uname`"
 _now=$(date +"%m_%d_%Y")
 _file="wp-data/data_$_now.sql"
 docker-compose exec db sh -c 'exec mysqldump "$MYSQL_DATABASE" -uroot -p"$MYSQL_ROOT_PASSWORD"' > $_file
-sed -i 1,1d $_file # Removes the password warning from the file
+if [[ $_os == "Darwin"* ]] ; then
+  sed -i '.bak' 1,1d $_file
+else
+  sed -i 1,1d $_file # Removes the password warning from the file
+fi