[gemfire] update to latest artifact.

closes #94

gemfire/pom.xml

@@ -15,7 +15,7 @@
     <dependency>
       <groupId>com.gemstone.gemfire</groupId>
       <artifactId>gemfire</artifactId>
-      <version>7.0.2</version>
+      <version>${gemfire.version}</version>
     </dependency>
     <dependency>
       <groupId>com.yahoo.ycsb</groupId>

pom.xml

@@ -54,6 +54,7 @@
     <accumulo.version>1.6.0</accumulo.version>
     <cassandra.version>1.2.9</cassandra.version>
     <cassandra.cql.version>1.0.3</cassandra.cql.version>
+    <gemfire.version>8.1.0</gemfire.version>
     <infinispan.version>7.1.0.CR1</infinispan.version>
     <openjpa.jdbc.version>2.1.1</openjpa.jdbc.version>
     <!--<mapkeeper.version>1.0</mapkeeper.version>-->