Run Coverity Scan builds on Travis CI

Trigger Coverity Scan whenever we push into the "coverity" branch.

Thanks to Jan Wagner for figuring out how to disable the Clang build
when using the Coverity Scan add-on.
.travis.yml: "install" vs. "before_install"

The Travis CI documentation says:

| - Run "before_install" commands
|
|   Use this to prepare the system to install prerequisites or
|   dependencies (e.g. "sudo apt-get update").
|
| - Run "install" commands
|
|   Use this to install any prerequisites or dependencies necessary to
|   run your build.

[ http://docs.travis-ci.com/user/build-configuration/ ]
.travis.yml: Stylistic change

Specify anything that could have multiple elements as a list.