fix(java check): put java check only in bin usage

Java check cannot be done in programmatic usage given the current
programmatic usage API.

`whereis` is asynchronous. while programmatic API is currently
"synchronous": `var server = require('selenium-standalone')(opts)`.

So for java check to work in programmatic usage it would require to
change API to:

```js
require('selenium-standalone')(opts, function(err, cb) {
  // err filled whether java is available or not, or selenium not
  // available/downloaded
})
```

I wonder if this work is really necessary, the main issue was the ENOENT
error with no particular information.

Luckily there's some work done in nodejs core to handle this case:
https://github.com/joyent/node/pull/7520

@jirit @Paul-E thoughts?