[SPARK-6544][build] Increment Avro version from 1.7.6 to 1.7.7

Fixes bug causing Kryo serialization to fail with Avro files in between stages.

https://issues.apache.org/jira/browse/AVRO-1476?focusedCommentId=13999249&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-13999249

Author: Dean Chen <deanchen5@gmail.com>

Closes #5193 from deanchen/SPARK-6544 and squashes the following commits:

813d4c5 [Dean Chen] [SPARK-6544][build] Increment Avro version from 1.7.6 to 1.7.7

pom.xml

@@ -141,7 +141,7 @@
     <ivy.version>2.4.0</ivy.version>
     <oro.version>2.0.8</oro.version>
     <codahale.metrics.version>3.1.0</codahale.metrics.version>
-    <avro.version>1.7.6</avro.version>
+    <avro.version>1.7.7</avro.version>
     <avro.mapred.classifier></avro.mapred.classifier>
     <jets3t.version>0.7.1</jets3t.version>
     <aws.java.sdk.version>1.8.3</aws.java.sdk.version>