pom: fix javadoc build on jdk12

- set source to jdk 7

per https://bugs.openjdk.java.net/browse/JDK-8212233

Author: srl295

.travis.yml

@@ -13,6 +13,7 @@ jdk:
   - openjdk9
   - openjdk10
   - openjdk11
+  - openjdk12
 
 before_install:
   - echo -n | openssl s_client -connect scan.coverity.com:443 | sed -ne '/-BEGIN CERTIFICATE-/,/-END CERTIFICATE-/p' | sudo tee -a /etc/ssl/certs/ca-

gp-cli/pom.xml

@@ -26,6 +26,9 @@
 			<plugin>
 				<groupId>org.apache.maven.plugins</groupId>
 				<artifactId>maven-javadoc-plugin</artifactId>
+                                <configuration>
+                                  <source>7</source>
+                                </configuration>
 			</plugin>
 
 			<plugin>

gp-maven-plugin/pom.xml

@@ -26,6 +26,9 @@
 			<plugin>
 				<groupId>org.apache.maven.plugins</groupId>
 				<artifactId>maven-javadoc-plugin</artifactId>
+                                <configuration>
+                                  <source>7</source>
+                                </configuration>
 			</plugin>
 
 			<plugin>

gp-res-filter/pom.xml

@@ -26,6 +26,9 @@
 			<plugin>
 				<groupId>org.apache.maven.plugins</groupId>
 				<artifactId>maven-javadoc-plugin</artifactId>
+                                <configuration>
+                                	<source>7</source>
+                                </configuration>
 			</plugin>
 			<plugin>
 				<groupId>org.apache.maven.plugins</groupId>