redis-brain: Remove default prefix from default Redis URL

The default 'hubot' prefix is explicitly handled by the line
calculating the 'prefix' variable.  It's redundant (not DRY) to also
specify this prefix in the default Redis URL.
redis-brain: Check if info.path is null before replacing strings

If the optional prefix path is not given, redis-brain currently dies
with:

  ERROR Unable to load /.../redis-brain: TypeError: Cannot call method 'replace' of null
    at module.exports (/.../redis-brain.coffee:28:12, <js>:13:24)
    at Robot.loadFile (/.../robot.coffee:216:9, <js>:162:24)
    at Robot.loadHubotScripts (/.../robot.coffee:243:7, <js>:195:28)
    at /.../hubot:98:15, <js>:99:30
    at fs.js:266:14
    at Object.oncomplete (fs.js:107:15)

because:

  Url.parse "redis://localhost:6379"

returns a null path:

  { protocol: 'redis:',
    slashes: true,
    auth: null,
    host: 'localhost:6379',
    port: '6379',
    hostname: 'localhost',
    hash: null,
    search: null,
    query: null,
    pathname: null,
    path: null,
    href: 'redis://localhost:6379' }

Avoid that by only calling info.path.replace if info.path is truthy.

Neither this implementation nor the one it replaces will let you
explicitly avoid setting a prefix, but that's probably not a big deal.
Merge pull request #1430 from wking/redis-brain-null-path

redis-brain: Check if info.path is null before replacing strings