Remove watch of Gemfile to avoid problems on (permanent) restarts.

Hi, I know you're searching for a maintainer, but this fix is easy and you could quickly think about it.

I always run into this problem:
As soon as I edit `Gemfile`, spork will restart itself.
Additionally, together with guard-rails, it tries restarts rails as well (as a side effect of restarting spork) which doesn't work because there are gems missing.
It always runs into this loop:

    23:49:27 - INFO - Reloading Spork for RSpec
    Could not find gem 'rails-i18n (>= 0) ruby' in the gems available on this machine.
    Run `bundle install` to install missing gems.
    23:49:57 - ERROR - Could not reload Spork server for RSpec after 30 seconds. I will continue waiting for a further 60 seconds.
    
    23:50:57 - ERROR - Could not reload Spork server for RSpec. Make sure you can use it manually first.
    
    23:52:52 - INFO - Restarting Rails...
    
    Could not find gem 'rails-i18n (>= 0) ruby' in the gems available on this machine.
    Run `bundle install` to install missing gems.
    23:53:25 - INFO - Rails NOT restarted, check your log files.

and continues.
Now imho, watching `Gemfile` doesn't make sense since it doesn't affect the app immediately. Only `Gemfile.lock` is important for that.
After removing `watch(Gemfile)` I have time to run `bundle install` which will, on success, immediately restart sport (and rails and so on).

Now you could also add guard-bundler at the top of spork to first create `Gemfile.lock`, which will trigger spork. (However, I don't want that for now).