Don't fail if './casperjs' symlink already exists.

[yjaaidi]

This can also be fixed by checking if the file exists first or by overwriting the existing file if any.

[xsellier]

To reproduce this we have to do a simple 'npm rebuild', then we get following error:

fs.js:730
return binding.symlink(preprocessSymlinkDestination(destination, type),
^
Error: EEXIST, file already exists '/home/xsellier/work/tmp/node_modules/grunt-casperjs/lib/casperjs/casperjs-master/bin/casperjs'
at Object.fs.symlinkSync (fs.js:730:18)
at IncomingMessage.unzipTheZippedFile (/home/xsellier/work/tmp/node_modules/grunt-casperjs/install.js:49:12)
at IncomingMessage.EventEmitter.emit (events.js:117:20)
at _stream_readable.js:920:16
at process._tickCallback (node.js:415:13)

[jbelis]

Any plan on merging this in?