Update bin/replace script for better portability

The existing implementation of the `replace` script does not work on
GNU/Linux due to differences in the implementation of `sed`. This change
reworks the process of the replacement to create temporary files,
apply the changes to the files, and then move them into place. It will
improve the script's portability for more OSes.

bin/replace

@@ -9,4 +9,8 @@ shift
 replace_with="$1"
 shift
 
-ag -l --nocolor "$find_this" "$@" | xargs sed -i '' "s/$find_this/$replace_with/g"
+items=$(ag -l --nocolor "$find_this" "$@")
+temp="${TMPDIR:-/tmp}/replace_temp_file.$$"
+for item in $items; do
+  sed "s/$find_this/$replace_with/g" "$item" > "$temp" && mv "$temp" "$item"
+done