enable Gradle wrapper validation

add back API that was accidentally deleted in refactor

Upgrading Travis CI to OpenJDK

Oracle JDK 8 has been failing to build ribbon PRs for several
months due to the license agreement.  Netflix moved to OpenJDK
over one year ago.

config: Misc cleanup

Bugfix/reduce property resolution

# Conflicts:
#	ribbon-core/src/main/java/com/netflix/client/config/ReloadableClientConfig.java

config: Fix property resolution when config not loaded

config: Ensure dynamic config is always loaded for defaults

config: Fix getIfSet

config: Property update behavior

Change the property value update semantics to more closely match the previous state before the refactoring.  Specifically, values are cached in a single map of key to final resolved values.

config: Fix isLoaded not set in the correct order

config: Simplify how overrides are applied

config: Fix excessive change notifications

config: fix config reload

When reloading default config reset the previous dynamic properties so the new client name is applied

config: Dynamic properties that are only scoped to the client (no default)

# Conflicts:
#	ribbon-core/src/main/java/com/netflix/client/config/ReloadableClientConfig.java

config: Property for mapping multiple property values with prefix

config: Method to get config if it's set without reading properties

feature: Fix returning Property defaults

The current mechanism for returning property default values is a bit awkward because of how property overrides are applied.  This change addresses that by adding an explicit `getOrDefault` api.

config: API to get a config's PropertyResolver