update install command for heroku

[dlionz]

Looks like $ brew install heroku-toolbelt throws an error now.

Heroku docs suggest $ brew install heroku/brew/heroku

Error: No available formula with the name "homebrew/core/heroku" ==> Searching for a previously deleted formula (in the last month)... Warning: homebrew/core is shallow clone. To get complete history run: git -C "$(brew --repo homebrew/core)" fetch --unshallow

Error: No previously deleted formula found. ==> Searching for similarly named formulae... Error: No similarly named formulae found.

[simeg]

@dlionz Thanks for contributing!