.travis.yml: Change CI make test to make test-full

- Change the continuous integration script to use 'make test-full'
instead of just 'make test' so that the diagnostic log output is
printed to stdout when a test fails.

- Change the continuous integration script to use
'./configure --enable-debug' instead of just './configure' so that the
memory analyzer will work during testing.

Prior to this change Travis used its default C test script:
./configure && make && make test