Add Adobe AIR installer

And also note the tentative syntax for
caveats/post-install notes. Which is
actually just my way of meta-saying:
‘This is an installer, not the shtuff!’
Fix Gimp shasum
Add new Cask: GnuCash
Update test cask

Test cask previously contained only attributes `url`, `homepage` and
`version`.  Add also `content_length` and `sha1`.
Merge pull request #128 from mkouhia/fix-gimp

Fix Gimp shasum
Merge pull request #129 from mkouhia/gnucash

Add new Cask: GnuCash
Merge pull request #121 from passcod/adobe-air

Add Adobe AIR installer
Merge pull request #130 from mkouhia/update-test-formula

Update test cask