chore(java): change java version to 11

Author: Jiankai Zheng

.travis.yml

@@ -1,6 +1,6 @@
 language: java
 jdk:
-  - openjdk15
+  - openjdk11
 
 after_success:
   - mvn test jacoco:report coveralls:report

lgtm.yml

@@ -1,4 +1,4 @@
 extraction:
   java:
     index:
-      java_version: 14
+      java_version: 11

pom.xml

@@ -182,8 +182,8 @@
     </dependencies>
 
     <properties>
-        <maven.compiler.target>15</maven.compiler.target>
-        <maven.compiler.source>15</maven.compiler.source>
+        <maven.compiler.target>11</maven.compiler.target>
+        <maven.compiler.source>11</maven.compiler.source>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
     </properties>
 </project>