prepare 5.7.3 release

[LaunchDarklyCI]

[5.7.3] - 2020-04-27

Changed:

• Previously, installing the SDK in an environment that did not have openssl would cause a failure at build time. The SDK still requires openssl at runtime, but this check has been removed because it caused the rake problem mentioned below, and because openssl is normally bundled in modern Ruby versions.

Fixed:

• The LDClient constructor will fail immediately with a descriptive ArgumentError if you provide a nil SDK key in a configuration that requires an SDK key (that is, a configuration that will require communicating with LaunchDarkly services). Previously, it would still fail, but without a clear error message. You are still allowed to omit the SDK key in an offline configuration. (#154)
• Removed a hidden dependency on rake which could cause your build to fail if you had a dependency on this SDK and you did not have rake installed. (#155)
• Previously a clause in a feature flag rule that used a string operator (such as "starts with") or a numeric operator (such as "greater than") could cause evaluation of the flag to completely fail and return a default value if the value on the right-hand side of the expression did not have the right data type-- for instance, "greater than" with a string value. The LaunchDarkly dashboard does not allow creation of such a rule, but it might be possible to do so via the REST API; the correct behavior of the SDK is to simply treat the expression as a non-match.