[FLINK-5039] Bump Avro version to 1.7.7.

This closes #2953.

pom.xml

@@ -220,13 +220,13 @@ under the License.
 			<dependency>
 				<groupId>org.apache.avro</groupId>
 				<artifactId>avro</artifactId>
-				<version>1.7.6</version>
+				<version>1.7.7</version>
 			</dependency>
 			
 			<dependency>
 				<groupId>org.apache.avro</groupId>
 				<artifactId>avro-ipc</artifactId>
-				<version>1.7.6</version>
+				<version>1.7.7</version>
 			</dependency>
 
 			<!-- Make sure we use a consistent commons-cli version throughout the project -->