prepare 4.2.0 release

[eli-darkly]

[4.2.0] - 2018-06-26

Added:

• New overloads of LDUser.Builder.custom and LDUser.Builder.privateCustom allow you to set a custom attribute value to any JSON element.

Changed:

• The client now treats most HTTP 4xx errors as unrecoverable: that is, after receiving such an error, it will not make any more HTTP requests for the lifetime of the client instance, in effect taking the client offline. This is because such errors indicate either a configuration problem (invalid SDK key) or a bug, which is not likely to resolve without a restart or an upgrade. This does not apply if the error is 400, 408, 429, or any 5xx error.
• During initialization, if the client receives any of the unrecoverable errors described above, the client constructor will return immediately; previously it would continue waiting until a timeout. The initialized() method will return false in this case.