GIRAPH-1055: Javadoc fails build with Java 8

GIRAPH-1055: Javadoc fails build with Java 8

Summary:
Java 8 javadoc has stricter checking, which results in mvn javadoc:javadoc failing:
Example:
100 errors
200 warnings
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Summary:
[INFO]
[INFO] Apache Giraph Parent ............................... SUCCESS [ 1.196 s]
[INFO] Apache Giraph Core ................................. FAILURE [ 9.583 s]

Test Plan:
[INFO] --- maven-javadoc-plugin:2.9:javadoc (default-cli) @ giraph-dist ---
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Summary:
[INFO]
[INFO] Apache Giraph Parent ............................... SUCCESS [  1.093 s]
[INFO] Apache Giraph Core ................................. SUCCESS [ 10.934 s]
[INFO] Apache Giraph Blocks Framework ..................... SUCCESS [  3.245 s]
[INFO] Apache Giraph Examples ............................. SUCCESS [  3.841 s]
[INFO] Apache Giraph Accumulo I/O ......................... SUCCESS [  2.048 s]
[INFO] Apache Giraph HBase I/O ............................ SUCCESS [  1.132 s]
[INFO] Apache Giraph HCatalog I/O ......................... SUCCESS [  3.053 s]
[INFO] Apache Giraph Gora I/O ............................. SUCCESS [  3.500 s]
[INFO] Apache Giraph Rexster I/O .......................... SUCCESS [  0.091 s]
[INFO] Apache Giraph Rexster Kibble ....................... SUCCESS [  1.276 s]
[INFO] Apache Giraph Rexster I/O Formats .................. SUCCESS [  3.193 s]
[INFO] Apache Giraph Distribution ......................... SUCCESS [  2.074 s]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 35.880 s
[INFO] Finished at: 2016-04-22T16:09:04-07:00
[INFO] Final Memory: 62M/1131M
[INFO] ------------------------------------------------------------------------

Reviewers: maja.kabiljo, sergey.edunov

Reviewed By: sergey.edunov

Differential Revision: https://reviews.facebook.net/D57105