.travis.yml: remove "make check" from main matrix

There are problems with unreliability in "make check" which still need
to be tracked down. As the tests are broadly the same for all targets if
added one explicit target to the matrix to run it. However this does
build all softmmu targets to ensure they at least "run"

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>