Fine-tuning for a few string operations

[elfring]

I find that a few string operations can be optimised in the function "launchVM".

• How do you think about to assign the result of the expression "execDir + std::string("/config.json")" to a variable?
• Would it be a bit more efficient to append data to the variable "execDir" instead of using the additional "jarFile"?

[badlogic]

I think we should aim for correctness and ease of use before we do microoptimizations. That being said, having a local var for execDir + std::string("config.json") makes total sense from a DRY perspective.

[badlogic]

Merged into #11