Improve the URL of "browser-sync-client.js"

I updated 2 points.

**1. Correct the script URL**
```
# wrong
<script async src="///browser-sync/browser-sync-client1.4.0.js "></script>

# correct
<script async src="/browser-sync/browser-sync-client.1.4.0.js"></script>
```

**2. Make "version" parameter optional**

The version number of "browser-sync-client.js" is not necessary in v0.7.0 or later. 
See: https://github.com/shakyShane/browser-sync/issues/82

When user specifies `version`, works as before. But it's optional.
I think this is more user-friendly.
```
# specify { version: '1.4.0' }
<script async src="/browser-sync/browser-sync-client.1.4.0.js"></script>

# omit version option
<script async src="/browser-sync/browser-sync-client.js"></script>
```