Standardize use of memegenerator config in futurama.coffee

Futurama meme script uses HUBOT_MEMEGENERATOR_USER and HUBOT_MEMEGENERATOR_PASS, where as the meme-generator script uses HUBOT_MEMEGENERATOR_USERNAME and HUBOT_MEMEGENERATOR_PASSWORD. If we standardize these variables, then you don't have to set your username and password a second time.