The answer is just add @dev
{
"require": {
"cartalyst/sentry": "2.0.*@dev"
},
}
You can read more about minimum stability settings here.
An alternative is to set your minimum-stability to dev, but tell composer you want to use stable whenever possible:
"minimum-stability": "dev",
"prefer-stable" : true
This basically means it will always use stable UNLESS there is no way to install a stable dependency, and therefore use dev.