Downgrade checkstyle version to be compatible with Java 1.7 (Tencent#94)

Author: tisonkun

codestyle/checkstyle.xml

@@ -57,6 +57,13 @@ page at http://checkstyle.sourceforge.net/config.html -->
         <property name="file" value="${checkstyle.suppressions.file}" default="suppressions.xml" />
     </module>
 
+    <!-- Allow use of comment to suppress javadocstyle -->
+    <module name="SuppressionCommentFilter">
+        <property name="offCommentFormat" value="CHECKSTYLE.OFF\: ([\w\|]+)"/>
+        <property name="onCommentFormat" value="CHECKSTYLE.ON\: ([\w\|]+)"/>
+        <property name="checkFormat" value="$1"/>
+    </module>
+
     <!-- All Java AST specific tests live under TreeWalker module. -->
     <module name="TreeWalker">
 
@@ -65,13 +72,6 @@ page at http://checkstyle.sourceforge.net/config.html -->
         -->
         <module name="Indentation"/>
 
-        <!-- Allow use of comment to suppress javadocstyle -->
-        <module name="SuppressionCommentFilter">
-            <property name="offCommentFormat" value="CHECKSTYLE.OFF\: ([\w\|]+)"/>
-            <property name="onCommentFormat" value="CHECKSTYLE.ON\: ([\w\|]+)"/>
-            <property name="checkFormat" value="$1"/>
-        </module>
-
         <module name="TodoComment">
             <!-- Checks that disallowed strings are not used in comments.  -->
             <property name="format" value="(FIXME)|(XXX)|(@author)" />

pom.xml

@@ -44,7 +44,7 @@
         <powermock.version>1.6.5</powermock.version>
         <exec.maven.version>1.6.0</exec.maven.version>
         <build.helper.maven.version>3.0.0</build.helper.maven.version>
-        <checkstyle.version>8.18</checkstyle.version>
+        <checkstyle.version>6.19</checkstyle.version>
         <protobuf.version>2.5.0</protobuf.version>
         <je.version>7.3.7</je.version>
     </properties>
@@ -108,7 +108,7 @@
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-checkstyle-plugin</artifactId>
-                <version>3.1.0</version>
+                <version>2.16</version>
                 <dependencies>
                     <dependency>
                         <groupId>com.puppycrawl.tools</groupId>