Sanitized up documentationtools doctests.

For purposes of running Abjad on Travis-CI, references to the experimental and
scoremanager packages have been commented out of doctests.
Merge pull request #451 from gkthiruvathukal/master

Updates to support Travis CI.
Simplified .travis.yml.
Updated LilyPond version in .travis.yml.

We traditionally test against the most recent development version of LilyPond.

Question: is there a link online that always points to the most recent version
of LilyPond?
Updated test for Python 3.
Added graphviz and imagemagick to .travis.yml.
Removed help2man and texlive from .travis.yml.

Until we begin testing the PDF doc-building process, we don't need these.
Updated README with proper Travis-CI badge.
Rearranged README.
Added `ajv api -M` to .travis.yml.
Removed unused toplevel scripts/ directory.