Ensure obfuscate_env_vars doesn't fail with invalid vars.

When there's an invalid entry in the .travis.yml, the API fails to
return anything unfortunately as this method fails loudly when the
secure env variable is a hash entry rather than a string.
Merge pull request #11 from travis-ci/mm-fix-obfuscate-env-vars

Ensure obfuscate_env_vars doesn't fail with invalid vars.