Set explicit version

Hi Louis. I'm trying to debug an issue with stanford-core-nlp included in a rails app, then also included in a ruby gem. I noticed that withouth the explicit rjb version requirement, when I "bundle" in the rails app, which specifies the gem version, the latest version as of this writing of rjb `1.4.9` is getting included instead of `1.4.3`. I tried to pin the version to 1.4.3, as I read elsewhere that version was tested. 

Unfortunately I'm still debugging the original issue, but I wanted to pass this along in case you wanted to consider pinning the rjb dependency to a specific version anyway. Personally I prefer this, as even the minor version of rjb could introduce bugs, but I leave it up to you as to whether you want to merge it or not.

Thanks.