2.2.5 release preparations

[akarnokd]

Version 2.2.5 - December 31, 2018 (Maven)

Documentation changes

• Pull 6344: Use correct return type in JavaDocs documentation in elementAtOrDefault.
• Pull 6346: Fix JavaDoc examples using markdown instead of @code.

Wiki changes

• Pull 6324: Java 8 version for Problem-Solving-Examples-in-RxJava.
• Pull 6343: Update Filtering Observables docs.
• Pull 6351: Updated java example in How-To-Use-RxJava.md file with java 8 version.

Other changes

• Pull 6313: Adding @NonNull annotation factory methods.
• Pull 6335: Replace indexed loop with for-each java5 syntax.

[akarnokd]

Closing via v2.2.5.