Avoid the need to use a git remote when checking if DB migrations should be run

Fixes GH-38

### How to test this?

1. Generate a new project with `pah` (make sure you choose to create the app on heroku)
2. Update the Gemfile to make `jumpup-heroku` point to this branch [`fr-fix-db-tasks-on-production`]
3. Update `config/initializers/jumpup_heroku.rb` with the production and staging app names
4.
```
# Setup staging app
STAGING="<app>-staging" && \
heroku apps:create $STAGING && \
heroku addons:add -a $STAGING heroku-postgresql && \
heroku addons:add -a $STAGING pgbackups:auto-month && \
heroku addons:add -a $STAGING loggly && \
heroku addons:add -a $STAGING rollbar

# Initial deploy
rake integrate && rake jumpup:heroku:deploy:production

# New migration
rails generate model post title:string && git add . && git commit -am 'Generate Posts model'
rake integrate # This should migrate the DB on staging
rake jumpup:heroku:deploy:production # This should migrate the DB on prod as well, fixing the bug

# Seeds update
echo 'Post.create!(title: "new")' > db/seeds.rb && git add . && git commit -am 'Update seeds'
rake integrate # This should run the seeds on staging
rake jumpup:heroku:deploy:production # This should run the seeds on prod

# This should not trigger any DB related tasks apart from the backup
echo 'class Dummy; end' > lib/dummy.rb && git add . && git commit -am 'Dummy class'
rake integrate && rake jumpup:heroku:deploy:production
```