Allow `version` to be a Fixnum

A recent commit by @jacknagel (a16c80add0f350501b3c9d4c0cf12c0010263178) changed how the Formula version is parsed, and restricted the accepted types of the version argument to `String`, `Hash` and `nil`.

Thing is, we freeze our version of ImageMagick to a older version  https://github.com/mxcl/homebrew/blob/834ce4a/Library/Formula/imagemagick.rb#L61 – whose version argument is the 3. Yes, a Fixnum.

So, to be compatible with (at least one) older formula, I’ve fixed the code to accept Fixnum versions.