[SPARK-20343][BUILD] Add avro dependency in core POM to resolve build failure...

[SPARK-20343][BUILD] Add avro dependency in core POM to resolve build failure in SBT Hadoop 2.6 master on Jenkins

## What changes were proposed in this pull request?

This PR proposes to add

```
      <dependency>
        <groupId>org.apache.avro</groupId>
        <artifactId>avro</artifactId>
      </dependency>
```

in core POM to see if it resolves the build failure as below:

```
[error] /home/jenkins/workspace/spark-master-test-sbt-hadoop-2.6/core/src/main/scala/org/apache/spark/serializer/GenericAvroSerializer.scala:123: value createDatumWriter is not a member of org.apache.avro.generic.GenericData
[error]     writerCache.getOrElseUpdate(schema, GenericData.get.createDatumWriter(schema))
[error]
```

https://amplab.cs.berkeley.edu/jenkins/view/Spark%20QA%20Test%20(Dashboard)/job/spark-master-test-sbt-hadoop-2.6/2770/consoleFull

## How was this patch tested?

I tried many ways but I was unable to reproduce this in my local. Sean also tried the way I did but he was also unable to reproduce this.

Please refer the comments in https://github.com/apache/spark/pull/17477#issuecomment-294094092

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #17642 from HyukjinKwon/SPARK-20343.

core/pom.xml

@@ -33,6 +33,10 @@
   <name>Spark Project Core</name>
   <url>http://spark.apache.org/</url>
   <dependencies>
+    <dependency>
+      <groupId>org.apache.avro</groupId>
+      <artifactId>avro</artifactId>
+    </dependency>
     <dependency>
       <groupId>org.apache.avro</groupId>
       <artifactId>avro-mapred</artifactId>