Fix dep exclusion: avro-ipc, not avro, depends on netty.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #763 from vanzin/netty-dep-hell and squashes the following commits:

dfb6ce2 [Marcelo Vanzin] Fix dep exclusion: avro-ipc, not avro, depends on netty.

pom.xml

@@ -496,18 +496,16 @@
         <groupId>org.apache.avro</groupId>
         <artifactId>avro</artifactId>
         <version>${avro.version}</version>
-        <exclusions>
-          <exclusion>
-            <groupId>io.netty</groupId>
-            <artifactId>netty</artifactId>
-          </exclusion>
-        </exclusions>
       </dependency>
       <dependency>
         <groupId>org.apache.avro</groupId>
         <artifactId>avro-ipc</artifactId>
         <version>${avro.version}</version>
         <exclusions>
+          <exclusion>
+            <groupId>io.netty</groupId>
+            <artifactId>netty</artifactId>
+          </exclusion>
           <exclusion>
             <groupId>org.mortbay.jetty</groupId>
             <artifactId>jetty</artifactId>