Remove FastStack reference from ReturnNullInsteadOfBoolean

[romani]

Usage of Checkstyle 6.7 + Sevntu Chekstyle 1.13.2 cause problem.

any reference to FastStack should be removed. See details at checkstyle/checkstyle#86

log:

BUILD FAILED
sevntu/checkstyle-samples/ant-project/build.xml:15: java.lang.NoClassDefFoundError: com/puppycrawl/tools/checkstyle/api/FastStack
    at com.github.sevntu.checkstyle.checks.coding.ReturnNullInsteadOfBoolean.<init>(ReturnNullInsteadOfBoolean.java:25)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:422)
    at java.lang.Class.newInstance(Class.java:442)
    at com.puppycrawl.tools.checkstyle.PackageObjectFactory.createObject(PackageObjectFactory.java:114)
    at com.puppycrawl.tools.checkstyle.PackageObjectFactory.doMakeObject(PackageObjectFactory.java:83)
    at com.puppycrawl.tools.checkstyle.PackageObjectFactory.createModule(PackageObjectFactory.java:135)
    at com.puppycrawl.tools.checkstyle.TreeWalker.setupChild(TreeWalker.java:163)
    at com.puppycrawl.tools.checkstyle.api.AutomaticBean.configure(AutomaticBean.java:171)
    at com.puppycrawl.tools.checkstyle.Checker.setupChild(Checker.java:160)
    at com.puppycrawl.tools.checkstyle.api.AutomaticBean.configure(AutomaticBean.java:171)
    at com.puppycrawl.tools.checkstyle.CheckStyleTask.createChecker(CheckStyleTask.java:353)
    at com.puppycrawl.tools.checkstyle.CheckStyleTask.realExecute(CheckStyleTask.java:280)
    at com.puppycrawl.tools.checkstyle.CheckStyleTask.execute(CheckStyleTask.java:243)
    at org.apache.tools.ant.UnknownElement.execute(UnknownElement.java:291)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:497)
    at org.apache.tools.ant.dispatch.DispatchUtils.execute(DispatchUtils.java:106)
    at org.apache.tools.ant.Task.perform(Task.java:348)
    at org.apache.tools.ant.Target.execute(Target.java:390)
    at org.apache.tools.ant.Target.performTasks(Target.java:411)
    at org.apache.tools.ant.Project.executeSortedTargets(Project.java:1399)
    at org.apache.tools.ant.Project.executeTarget(Project.java:1368)
    at org.apache.tools.ant.helper.DefaultExecutor.executeTargets(DefaultExecutor.java:41)
    at org.apache.tools.ant.Project.executeTargets(Project.java:1251)
    at org.apache.tools.ant.Main.runBuild(Main.java:809)
    at org.apache.tools.ant.Main.startAnt(Main.java:217)
    at org.apache.tools.ant.launch.Launcher.run(Launcher.java:280)
    at org.apache.tools.ant.launch.Launcher.main(Launcher.java:109)
Caused by: java.lang.ClassNotFoundException: com.puppycrawl.tools.checkstyle.api.FastStack
    at org.apache.tools.ant.AntClassLoader.findClassInComponents(AntClassLoader.java:1361)
    at org.apache.tools.ant.AntClassLoader.findClass(AntClassLoader.java:1311)
    at org.apache.tools.ant.AntClassLoader.loadClass(AntClassLoader.java:1064)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
    ... 33 more

[romani]

fixed , released at 1.13.3