website: footer.html: use 1.7.1 release of clipboard.js

The clipboard javascript code causes an error:

Uncaught TypeError: Illegal constructor
    at buildroot.js:58

Which is caused by a recent change in upstream clipboard.js:

https://github.com/zenorocha/clipboard.js/releases/tag/v2.0.0

Changes constructor from new Clipboard() to new ClipboardJS()

The issue was that we were using the master version of clipboard.js instead
of a specific release.  Fix it by using the 1.7.1 version (latest release
before the constructor change) instead.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/website/footer.html

@@ -1,6 +1,6 @@
 	<script src="https://code.jquery.com/jquery-3.1.1.min.js"></script>
 	<script src="https://oss.maxcdn.com/bootstrap/3.3.7/js/bootstrap.min.js"></script>
-	<script src="https://cdn.rawgit.com/zenorocha/clipboard.js/master/dist/clipboard.min.js"></script>
+	<script src="https://cdn.rawgit.com/zenorocha/clipboard.js/v2.0.0/dist/clipboard.min.js"></script>
 	<script type="text/javascript" src="js/buildroot.js"></script>
 </body>
 </html>