Check the value of the webhook as rails is booting and throw a message and an error if it is invalid

[codelitt]

Prevents having issues in a running app.

[Lockyy]

If we're going to be throwing an error for not setting a webhook we can't use that as a way to turn off message sending so we should add a setting to development.rb or test.rb that by default turns off Slacked.post. #11

[codelitt]

@kaiomagalhaes @Lockyy close?

[kaiomagalhaes]

@codelitt We only need to make sure that we will be checking the environment before throwing the error