install-local-mac.sh made more readable

hi Zef,

first of all i really like your zed project! while i was checking what the mac installer does, i saw a loads of lines like:

```
echo "something" > output_file
echo "second line" >> output_file
echo "third line with $VARIABLE" >> output_file
```

it was quite hard to read, and could be made more human friendly with bash's [here-documents](http://www.tldp.org/LDP/abs/html/here-docs.html) feature

```
cat >output_file <<EOF
something
second line
third line with $VARIABLE
EOF
```

so its a pure cosmetics change, i just wanted make this script look nice.