Fix for suddenly appeared chef error

Probably due to some Chef-Environment Setup, ```chef_gem "BibOpsworks"``` suddenly fails. This PR fixes it.

```[2014-01-30T10:03:40+00:00] FATAL: ArgumentError: ruby_block[Compile Custom OpsWorks Run List] (opsworks_custom_cookbooks::execute line 3) had an error: ArgumentError: chef_gem[BibOpsworks] (easybib::setup line 13) had an error: ArgumentError: Illformed requirement [""]```