Set CFLAGS='-w', CCFLAGS='-w'

[BanzaiMan]

To suppress compiler warnings.

This PR introduces a new environment variable type, Defaults, that are not strictly Travis built-ins, but are common environment variables that users may define and subsequently override with their configurations (be in Settings panel, or in .travis.yml).

[BanzaiMan]

Tested: https://staging.travis-ci.org/BanzaiMan/travis_staging_test/builds/727771 (See, in particular, https://staging.travis-ci.org/BanzaiMan/travis_staging_test/builds/727771#L249-L250)

This shows how .travis.yml overrides the default value: https://staging.travis-ci.org/BanzaiMan/travis_staging_test/builds/727775#L26

[cotsog]

Hmmm could we declare this environment variable only when we call nvm ? Maybe it would be less disruptive this way?

[BanzaiMan]

@cotsog The wider change was suggested by @joshk.