replace WikipediaSystemAdmin with SinglePartitionSystemAdmin

WikipediaSystemAdmin doesn't override getLastOffsets() so just replacing it with SinglePartitionSystemAdmin.

Fixes:
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.1:compile (default-compile) on project samza-wikipedia: Compilation failure
[ERROR] /Projects/samza/hello-samza/samza-wikipedia/src/main/java/samza/examples/wikipedia/system/WikipediaSystemAdmin.java:[32,8] samza.examples.wikipedia.system.WikipediaSystemAdmin is not abstract and does not override abstract method getLastOffsets(java.util.Set<java.lang.String>) in org.apache.samza.system.SystemAdmin