Allow a full make check on Travis

Allow a full make check on Travis

Some Travis instances allow more CPU time. On these we can perform the
full 'make check'.

This patch allows this longer `make check`. To enable it you need to
go into your Travis configuration and add a TRAVIS_FULL_CHECK
environment variable.

If you don't add this environment, the shorter make check_light is
still run.

Signed-off-by: Michael Neuling <mikey@neuling.org>