update checkstyle to support java 8 syntax

With default version getting errors like:

```
[ant:checkstyle] /home/travis/build/Netflix/spectator/spectator-api/src/main/java/com/netflix/spectator/api/AbstractRegistry.java:105:11: unexpected token: aggr
[ant:checkstyle] /home/travis/build/Netflix/spectator/spectator-api/src/main/java/com/netflix/spectator/api/AbstractRegistry.java:108:5: Got an exception - expecting EOF, found 'if'
```

Updating to latest 6.8.1 fails with:

https://discuss.gradle.org/t/checkstyle-6-8-release-brake-compatibility-with-gradle-checkstyle-plugin/10375

So using 6.7. Note that RedundantThrows check was
removed:

http://stackoverflow.com/questions/27919444/java-checkstyle-redundant-throws

build.gradle

@@ -73,6 +73,7 @@ subprojects {
   }
 
   checkstyle {
+    toolVersion = '6.7'
     ignoreFailures = false 
     configFile = rootProject.file('codequality/checkstyle.xml')
     sourceSets = [sourceSets.main]

codequality/checkstyle.xml

@@ -143,11 +143,6 @@
         </module>-->
         <module name="MissingSwitchDefault"/>
         <!-- Problem with finding exception types... -->
-        <module name="RedundantThrows">
-          <property name="allowUnchecked" value="true"/>
-          <property name="suppressLoadErrors" value="true"/>
-          <property name="severity" value="info"/>
-        </module>
         <module name="SimplifyBooleanExpression"/>
         <module name="SimplifyBooleanReturn"/>