Update hacking instructions to use bundler prefix

If we're using bundler and rake, then you can end 
up with frustrating conflicts if your rake uses 
the system rake instead the version specified by
bundler. Using the bundler version at all times 
avoids this.

HACKING.md

@@ -51,7 +51,7 @@ Now, install required gems:
 
 3.  Create your local config files:
 
-        $ rake oco:generate_config
+        $ bundle exec rake oco:generate_config
 
 4.  Set up your database connection settings, by editing config/database.yml as desired.
 
@@ -64,7 +64,7 @@ Now, install required gems:
 
 5.  (Create and) migrate the database:
 
-        $ rake db:setup
+        $ bundle exec rake db:setup
 
 ## Running
 
@@ -74,10 +74,10 @@ Now, install required gems:
 
 2.  (Optional) In another shell, start the job server (which sends emails and closes proposals):
 
-        $ script/delayed_job run    # run in foreground
-        $ script/delayed_job start  # start as daemon (check log/delayed_job.log)
-        $ rake jobs:work            # foreground, from rake
-        $ rake jobs:clear           # clear queue
+        $ script/delayed_job run                # run in foreground
+        $ script/delayed_job start              # start as daemon (check log/delayed_job.log)
+        $ bundle exec rake jobs:work            # foreground, from rake
+        $ bundle exec rake jobs:clear           # clear queue
 
 3.  Visit `http://localhost:3000/`.
 
@@ -91,23 +91,23 @@ the founding steps manually.
 
 To do this, run the `oco:dev:create_association` rake task:
 
-  $ rake oco:dev:create_association
+  $ bundle exec rake oco:dev:create_association
 
 ## Updating
 
 When you pull updates from the repository, you may need to update your gem bundle (if `Gemfile.lock` has changed) and/or migrate your database (if `db/migrate` has changed):
 
     $ bundle install
-    $ rake db:migrate
+    $ bundle exec rake db:migrate
 
 These commands will safely no-op if no changes are necessary.
 
 ## Contributing
 
 Before you submit any patches make sure that no tests fail:
 
-      $ rake spec
-      $ rake cucumber
+      $ bundle exec rake spec
+      $ bundle exec rake cucumber
 
 To submit a patch, you have several options: