min-lein-version doesn't work with multi-line

The Clojure buildpack ignores `:min-lein-version` in `project.clj` unless it's all on one line. The following works:

```clj
:min-lein-version "2.5.0"
```

While the multi-line version, which is valid Clojure, does not:

```clj
:min-lein-version
"2.5.0"
```


