Make erlalchemy optional

[murphyke]

Maybe these changes:

• Remove ERAlchemy from requirements.txt
• erd.write dynamically imports eralchemy or raises ImportError
• create_erd_route invokes render_template('erd_500.html'), 500 on ImportError
• Dockerfile explicitly installs ERAlchemy
• CircleCI config explicitly installs ERAlchemy
• Tests become resilient to presence or absence of ERAlchemy -- this adds flexibility and is safe because CircleCI performs the full tests.