Do not reuse baseDir local var if working directory changes.

This change checks to confirm that `process.cwd` matches what it was when
`baseDir` was cached.  If it has changed, we need to setup
`baseDir` again.

----

Before this change, the following would fail:

```javascript var fs = require('fs') var quickTemp = require('quick-temp')

var somePathObj = {} fs.mkdirSync('somePath') process.chdir('somePath') 
quickTemp.makeOrRemake(somePathObj, 'tmpDestDir')

var someOtherPathObj = {} fs.mkdirSync('someOtherPath') 
process.chdir('someOtherPath') quickTemp.makeOrRemake(someOtherPathObj,
'tmpDestDir')
```

This fails because the local `baseDir` var is set/cached with a value that
references `tmp`, but once `process.cwd` changes we don't check for the
existence of `tmp/` again.