Upgrade to Error Prone to 2.3.4

2.3.4 is the latest supported by p-c-j-ep, as after google/error-prone@9eb414cc1
ErrorProneCompiler is no longer delivered with EP, and is not present in next
version 2.4.0.

This closes #75

Author: pzygielo

plexus-compilers/plexus-compiler-javac-errorprone/pom.xml

@@ -29,7 +29,7 @@
     <dependency>
       <groupId>com.google.errorprone</groupId>
       <artifactId>error_prone_core</artifactId>
-      <version>2.3.2</version>
+      <version>2.3.4</version>
     </dependency>
   </dependencies>