standardize Cask (capitalized) when used as noun

* was already done, but inconsistently
* this style follows homebrew Formula
* covers user-facing messages, test titles, comments
* some related minor orthography is included, such
  as the consistent spelling of our project name as
  "homebrew-cask"
* grammar nits
Merge pull request #6557 from rolandwalker/capitalize_cask

standardize Cask (capitalized) when used as noun