Locking mongo gem to specific version

It seems like after an [update to the mongo ruby gem](https://github.com/mongodb/mongo-ruby-driver/releases), unable to run the recipe successfully on Ubuntu 12.04. Just am getting:

```sh
================================================================================       
Error executing action `install` on resource 'chef_gem[mongo]'       
================================================================================       
       
       
Gem::Installer::ExtensionBuildError       
-----------------------------------       
ERROR: Failed to build gem native extension.       
       
        /opt/chef/embedded/bin/ruby extconf.rb       
checking for sasl/sasl.h... no       
checking for sasl_version() in -lsasl2... no       
creating Makefile       
       
make       
compiling csasl.c       
csasl.c:16:23: fatal error: sasl/sasl.h: No such file or directory       
compilation terminated.       
make: *** [csasl.o] Error 1       
       
       
Gem files will remain installed in /opt/chef/embedded/lib/ruby/gems/1.9.1/gems/mongo-1.11.0 for inspection.       
Results logged to /opt/chef/embedded/lib/ruby/gems/1.9.1/gems/mongo-1.11.0/ext/csasl/gem_make.out
```

This change just locks it to a specific version that works, though ideally we'd want to eventually figure out why the change broke the cookbook.