[REEF-1766] Updated Avro from 1.7.7 to 1.8.1 for entire REEF.

JIRA: [REEF-1766](https://issues.apache.org/jira/browse/REEF-1768)
This closes  #1285
diff --git a/pom.xml b/pom.xml
index b17e7ae..ed39250 100644
--- a/pom.xml
+++ b/pom.xml
@@ -51,7 +51,7 @@
         <bundle.snappy>false</bundle.snappy>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
         <hadoop.version>2.6.0</hadoop.version>
-        <avro.version>1.7.7</avro.version>
+        <avro.version>1.8.1</avro.version>
         <jetty.version>6.1.26</jetty.version>
         <jackson.version>1.9.13</jackson.version>
         <protobuf.version>2.5.0</protobuf.version>