Added development group to Gemfile

Hi,

I introduced a separate group `:development` into the `Gemfile` to remove some gems from the `:test` group.

The original reason is that running `rake spec` on the master branch segfaults for me with `ruby-1.9.3-p392 [ x86_64 ]` (using RVM):

```
/var/lib/jenkins/.rvm/gems/ruby-1.9.3-p392@errbit-master/gems/better_errors-0.7.0/lib/better_errors/core_ext/exception.rb:9: [BUG] Segmentation fault
ruby 1.9.3p392 (2013-02-22 revision 39386) [x86_64-linux]
```

Some investigation lead to `better_errors` being the potential reason, as is documented in https://github.com/charliesome/better_errors/issues/61.

While removing the gem from the `:test` group does not solve the issue, the documentation of `better_errors` also recommends to just add it to the `:development` group. I also verified that the specs still pass.

If someone is interested in the stack trace, do not hesitate to ask me.