Set WARNINGS=1 instead of CHPL_DEVELOPER in nightly.

CHPL_DEVELOPER was added to nightly in commit 06088fc9. Based on internal
notes, it appears CHPL_DEVELOPER was added to enable additional C compiler
warnings. With pull request #594 those warnings can now be enabled by setting
WARNINGS=1 when calling make.

It is also worth mentioning that start_test explicitly unsets CHPL_DEVELOPER
and adds --cc-warnings flag to the compopts. So, start_test is already running with
backend C compiler warnings.

Also add an additional test configuration to travis. In addition to testing with
CHPL_DEVELOPER=true (developer mode), also test in "nightly test mode". "nightly
test mode" does not set CHPL_DEVELOPER, but it does set DEBUG=0
WARNINGS=1 OPTIMIZE=1 when calling make.