Enable ErrorProne in the pom.xml

Use version 2.0.5 of ErrorProne. From version 2.0.6 JDK 8 is
required [1].

[1] https://github.com/google/error-prone/issues/369

Change-Id: Ib4fa6514d09ea6c3ea758ce08383627eca24da57
1 file changed