Removing one of two instances of 'rake' in Gemfile

I was getting weary of seeing this message every time I ran Bundler:
```
Your Gemfile lists the gem rake (>= 0) more than once.
You should probably keep only one of them.
While it's not a problem now, it could cause errors if you change the version of just one of them later.
```
Rake is included both in the main list of gems and in the development group. I opted to remove it from the development group since I believe admins as well as developers will want to be able to run some Rake tasks.