Improves the messaging of the heroku generator

I set my heroku config like this `heroku config:set RELEASE_KEY1=blah` (notice the equal sign) and I was getting weird errors on heroku deploy like below. I also tried with and without quotes.

```
padding check failed
/.rvm/gems/ruby-2.0.0-p247/bundler/gems/symmetric-encryption-a0d55cf6e32d/lib/symmetric_encryption/symmetric_encryption.rb:441:in `private_decrypt'
/.rvm/gems/ruby-2.0.0-p247/bundler/gems/symmetric-encryption-a0d55cf6e32d/lib/symmetric_encryption/symmetric_encryption.rb:441:in `cipher_from_config'
/.rvm/gems/ruby-2.0.0-p247/bundler/gems/symmetric-encryption-a0d55cf6e32d/lib/symmetric_encryption/symmetric_encryption.rb:351:in `block in read_config'
/.rvm/gems/ruby-2.0.0-p247/bundler/gems/symmetric-encryption-a0d55cf6e32d/lib/symmetric_encryption/symmetric_encryption.rb:351:in `collect'
/.rvm/gems/ruby-2.0.0-p247/bundler/gems/symmetric-encryption-a0d55cf6e32d/lib/symmetric_encryption/symmetric_encryption.rb:351:in `read_config'
/.rvm/gems/ruby-2.0.0-p247/bundler/gems/symmetric-encryption-a0d55cf6e32d/lib/symmetric_encryption/symmetric_encryption.rb:258:in `load!'
/.rvm/gems/ruby-2.0.0-p247/bundler/gems/symmetric-encryption-a0d55cf6e32d/lib/symmetric_encryption/railtie.rb:40:in `block in <class:Railtie>'
/.rvm/gems/ruby-2.0.0-p247/gems/activesupport-4.0.2/lib/active_support/lazy_load_hooks.rb:36:in `call'
/.rvm/gems/ruby-2.0.0-p247/gems/activesupport-4.0.2/lib/active_support/lazy_load_hooks.rb:36:in `execute_hook'
/.rvm/gems/ruby-2.0.0-p247/gems/activesupport-4.0.2/lib/active_support/lazy_load_hooks.rb:45:in `block in run_load_hooks'
/.rvm/gems/ruby-2.0.0-p247/gems/activesupport-4.0.2/lib/active_support/lazy_load_hooks.rb:44:in `each'
/.rvm/gems/ruby-2.0.0-p247/gems/activesupport-4.0.2/lib/active_support/lazy_load_hooks.rb:44:in `run_load_hooks'
/.rvm/gems/ruby-2.0.0-p247/gems/railties-4.0.2/lib/rails/application.rb:67:in `inherited'
```

However, if I set the heroku config like this `heroku config:set RELEASE_KEY1:blah` (notice the colon instead of equal sign), then everything worked fine. This PR changes the generator to stop people from making this mistake.