Run Coverity Scan builds on Travis CI

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