Upgrade to Babel 6

[Widdershin]

$ ./node_modules/markdown-doctest/bin/cmd.js
/Users/nickj/Projects/cycle-animation-driver/node_modules/markdown-doctest/node_modules/babel/node_modules/babel-core/lib/transformation/file/options/option-manager.js:126
      if (!option) this.log.error("Unknown option: " + alias + "." + key, ReferenceError);
                           ^

TypeError: Cannot read property 'error' of undefined
    at OptionManager.mergeOptions (/Users/nickj/Projects/cycle-animation-driver/node_modules/markdown-doctest/node_modules/babel/node_modules/babel-core/lib/transformation/file/options/option-manager.js:126:28)
    at OptionManager.addConfig (/Users/nickj/Projects/cycle-animation-driver/node_modules/markdown-doctest/node_modules/babel/node_modules/babel-core/lib/transformation/file/options/option-manager.js:107:10)
    at OptionManager.findConfigs (/Users/nickj/Projects/cycle-animation-driver/node_modules/markdown-doctest/node_modules/babel/node_modules/babel-core/lib/transformation/file/options/option-manager.js:168:35)
    at OptionManager.init (/Users/nickj/Projects/cycle-animation-driver/node_modules/markdown-doctest/node_modules/babel/node_modules/babel-core/lib/transformation/file/options/option-manager.js:229:12)
    at compile (/Users/nickj/Projects/cycle-animation-driver/node_modules/markdown-doctest/node_modules/babel/node_modules/babel-core/lib/api/register/node.js:117:22)
    at normalLoader (/Users/nickj/Projects/cycle-animation-driver/node_modules/markdown-doctest/node_modules/babel/node_modules/babel-core/lib/api/register/node.js:199:14)
    at Object.require.extensions.(anonymous function) [as .js] (/Users/nickj/Projects/cycle-animation-driver/node_modules/markdown-doctest/node_modules/babel/node_modules/babel-core/lib/api/register/node.js:216:7)
    at Module.load (module.js:355:32)
    at Function.Module._load (module.js:310:12)
    at Module.require (module.js:365:17)