.travis.yml: Rework Travis matrix

At the moment the .travis.yml implicitly constructs a build matrix
with the two compiler options.  In future we want to add more build
options for wider testing: different base distro, more compiler
versions, etc.  However, a fair few of the possible combinations have
various problems meaning we don't want to test them routinely.

So, this reworks from implicitly constructing the matrix to using
matrix: include: options to explicitly build the options we want.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>