MRUBY_VERSION should be only numbers separated by dot, like JRUBY_VERSION or Rubinius::VERSION

JRUBY_VERSION and Rubinius::VERSION are as follows:
```sh
$ jruby -e 'p JRUBY_VERSION'
"1.7.4"
$ rbx -e 'p Rubinius::VERSION'
"2.2.3"
```
They are same fomrat as RUBY_VERSION:

```sh
$ ruby -e 'p RUBY_VERSION'
"2.1.0"
```

So IMHO, it's better to use same format for MRUBY_VERSION, like "1.0.0", not "v1.0.0".