Update java version to 1.7 of sentinel-core

Signed-off-by: Carpenter Lee <hooleeucas@163.com>

pom.xml

@@ -51,8 +51,8 @@
 
         <!-- Build -->
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
-        <java.source.version>1.6</java.source.version>
-        <java.target.version>1.6</java.target.version>
+        <java.source.version>1.7</java.source.version>
+        <java.target.version>1.7</java.target.version>
         <java.encoding>UTF-8</java.encoding>
         <maven.compiler.version>3.8.0</maven.compiler.version>
         <maven.surefire.version>2.22.1</maven.surefire.version>