Mark development dependencies as such...
Mark development dependencies as such...

Avoids pulling down development depdencies for celluloid users.

Before:
```
...
  specs:
    celluloid (0.17.1.1)
      bundler
      celluloid-essentials
      celluloid-extras
      celluloid-fsm
      celluloid-pool
      celluloid-supervision
      dotenv
      nenv
      rspec-logsplit (>= 0.1.2)
      timers (~> 4.0.0)
...
```

After:
```
...
  specs:
    celluloid (0.17.5.pre)
      celluloid-essentials
      celluloid-extras
      celluloid-fsm
      celluloid-pool
      celluloid-supervision
      timers (~> 4.0.0)
...
```

celluloid/framework