Default value of ES_SCHEME should not have quotes

Currently, if this image is run without passing any ES_SCHEME, the value of 'elasticsearch' in the JSON ends up being: ""http://""+window.location.hostname+":9200" (with quotes).

The default value of ES_SCHEME becomes "http" (with quotes), which is wrong; it should not include the quotes.

This pull request removes the quotes and fixes the problem.