Deleted the Emacs configuration file from git repo.

This file is not required to be on the public repository.

[skip ci].
Minor cleanup of .gitignore.

[skip ci].
Removed the 'develop' branch from travis builds.

[skip ci]
Updated the tree_deps.rb to contain all require stmts.

This cleans up the require block in the main `tree.rb' file.
Removed the restriction on the rubygems version.

This was originally added in commit 6b4bec77 to get around the issue described
at https://github.com/travis-ci/travis-ci/issues/1793.

The issue seems to now have been resolved with the release of rubygems 2.2.1
Fix for Issue #24. Allow duplicates in non-sibling nodes.

The current code was too restrictive about duplicate nodes in the tree, whereas
the implementation requires only sibling node names to be unique.

Note that the uniqueness is defined (currently) by the node name.