Add org.apache.httpcomponents:(httpcore, httpclient) to dependency management

This closes #1301.

flink-runtime/pom.xml

@@ -78,7 +78,7 @@ under the License.
 		<dependency>
 			<groupId>org.apache.httpcomponents</groupId>
 			<artifactId>httpclient</artifactId>
-			<version>4.2</version>
+			<!-- Version is set in root POM -->
 		</dependency>
 
 		<dependency>

pom.xml

@@ -339,6 +339,18 @@ under the License.
 				<artifactId>netty-all</artifactId>
 				<version>4.0.31.Final</version>
 			</dependency>
+
+			<dependency>
+				<groupId>org.apache.httpcomponents</groupId>
+				<artifactId>httpcore</artifactId>
+				<version>4.2</version>
+			</dependency>
+
+			<dependency>
+				<groupId>org.apache.httpcomponents</groupId>
+				<artifactId>httpclient</artifactId>
+				<version>4.2</version>
+			</dependency>
 		</dependencies>
 	</dependencyManagement>