Using the absolute path to the JAR file

This commit is irrelevant by itself, but it allows jruby-memcached to be loaded correctly when it is included in a JAR file built with [warbler](https://github.com/jruby/warbler).

Without this fix, the following exception is thrown when we try to run the JAR file.

```
> java -jar /tmp/warble.jar

LoadError: no such file to load -- jar:file:/tmp/warble.jar!/gems/jruby-memcached-0.5.2/lib/../target/spymemcached-ext-0.0.1
```

In other cases, the change is harmless. So please consider merging it. Thanks.