Rather than incrementing an index each time we push, we now impose an indexing based on the repo structure. The version of the prerelease gem is defined to be the number of commits since the stable branch (precisely, the number of commits in `git log stable..master --first-parent`). This means that when someone `rake install`s Haml from a Git repo, the gem is installed with the proper version number.
Conflicts: doc-src/HAML_CHANGELOG.md lib/sass/tree/for_node.rb lib/sass/tree/if_node.rb lib/sass/tree/mixin_def_node.rb lib/sass/tree/mixin_node.rb lib/sass/tree/node.rb lib/sass/tree/root_node.rb lib/sass/tree/rule_node.rb lib/sass/tree/while_node.rb test/sass/engine_test.rb
…ther. Closes gh-314