avoid sub-classing ThreadLocal since it will leak in envs such as .wars

e.g. on Tomcat a Warbled up with this change (tested 1.7.13) allows to avoid the following warning(s) : 
```
INFO: Undeploying context [/hellowarld]
Jun 27, 2014 9:08:32 AM org.apache.catalina.loader.WebappClassLoader checkThreadLocalMapForLeaks
SEVERE: The web application [/hellowarld] created a ThreadLocal with key of type [org.jruby.util.Sprintf$2] (value [org.jruby.util.Sprintf$2@7cd67450]) and a value of type [java.util.HashMap] (value [{en_US=java.text.DecimalFormatSymbols@1073a}]) but failed to remove it when the web application was stopped. Threads are going to be renewed over time to try and avoid a probable memory leak.
Jun 27, 2014 9:08:32 AM org.apache.catalina.loader.WebappClassLoader checkThreadLocalMapForLeaks
SEVERE: The web application [/hellowarld] created a ThreadLocal with key of type [org.joni.StackMachine$1] (value [org.joni.StackMachine$1@636e7037]) and a value of type [java.lang.ref.WeakReference] (value [java.lang.ref.WeakReference@67895de0]) but failed to remove it when the web application was stopped. Threads are going to be renewed over time to try and avoid a probable memory leak.
Jun 27, 2014 9:08:32 AM org.apache.catalina.loader.WebappClassLoader checkThreadLocalMapForLeaks
```

related issue at https://github.com/jruby/joni/pull/11