Bugfix respect useCamelCaseFlagKeys in subscribeToChanges

Fixes the issue where subscribeToChanges always uses camel case flag keys even when useCamelCaseFlagKeys is false.

Added useCamelCaseFlagKeys option

Added a config option to turn off camel case keys.

Upgrade ld js client to launchdarkly-js-client-sdk

This release upgrades the js client package to the newly renamed launchdarkly-js-client-sdk.

Added subscribe option.

Provide means to opt-out of automatic live flag updates by adding a subscribe option in the init config.

Fixed deprecated default export warning and upgrades

• Fixed #22 ldclient-js warning re deprecated default export

[LaunchDarkly] "default export" is deprecated, please use "named LDClient export"

• Npm package upgrades

Make init flags optional

The flags argument is now no longer mandatory when initialising ld-redux. If you don't specify this argument, ld-redux will subscribe to all flags in your launch darkly environment.

If you specify the argument (this is the previous behaviour), then ld-redux will only subscribe to those flags specified.

This is a non-breaking change, no action required to upgrade to this version from v3.1.2.

Refer to GH-21 for the initial motivation for this change.