Update appcast script for python3
Latest objective-git
Latest objective-git: API changes

- [GTRepo lookupObjectByRevParse:error:] -> - [GTRepo lookUpObjectByRevParse:error:]
Sidestep error due to wrong class in the submodules collection

Remove PBGitSubmodule for now; just use GTSubmodule.
Merge remote-tracking branch 'refs/remotes/origin/master'
Fix work dir discovery for latest objective-git API
Fix segfault when forcing commit after a hook fails
Add requirements for build script
Fix import case in appcast.py
Fix appcast.py script under Python 3 and update the template
Late updates to the change log for 0.15.1869
Update README
Fix #310 - diff highlighter breaks on some strings

Strings like “constructor” occur as properties on JavaScript objects by default:

```
var x = new Object();
x[“constructor”] // Object
```
fixes #321: Word diff highlighting breaks for swapped phrases
Merge pull request #325 from muhqu/gitx-dev-issue/321

fixes #321: Word diff highlighting breaks for swapped phrases
#315 Generate lane colours on a HSB ring, much brighter and more saturated than the Solarized scheme
#315 Tone down the drop-shadow on ref labels

They still look a lot better than the solid outline, especially on Retina; so I’m hoping this helps with the visual weighting of the readability problem.
#315 lighter colours for ref labels
Merge remote-tracking branch 'refs/remotes/origin/master'
#315 Try bumping the font size of ref labels