Fix for arangodb 3.3 - rely on JSON.parse() instead of string parsing

[voondo]

Although arangochair works fine with arangodb 3.1, I had errors with 3.3. It seems the format changed a bit and the current string parser fails.

Here is a fix relying on JSON.parse() instead of parsing the string which seems more robust against format changes.

[yerboogieman]

Hi @voondo, my PR entitled "Emit separate events for inserts and updates" also has code to use JSON.parse() instead of relying on index-based string parsing. Perhaps we could merge our pull requests into one somehow so they do not conflict?