Add quotes around environment variable values when generating environment_hash

With something like this:

```ruby
with upload_url: "http://foo.com?bar=foo&bar=foo" do
  ...
end
```

The `environment_string` generates a string that looks like this: `UPLOAD_URL=http://foo.com?bar=foo&bar=foo` which is invalid because it contains the ampersand (`&`). The solution I found was to wrap the values in quotes.

Was this an acceptable solution?