Travis: Fix failing demos tests

[apsdehal]

Demos tests for travis have been failing for quite long.
Ref: https://travis-ci.org/jquery/jquery-mobile/jobs/313129859

It seems like a simple fix which will require npm install -g casper.

See: SpookyJS/SpookyJS#206

[Bachstelze]

How can on the same time fail the demo tests and build everything on the master except demos?
Nevertheless, in both the ppa from launchpad can't be accessed. Perhaps travis got the wrong source list?

[ecadius]

The rebuild with casper passes all tests: https://travis-ci.com/ecadius/jquery-mobile/builds/99162130
Update .travis.yml
ecadius@c7bbcaf