Update master Travis to Trusty

This gets us a newer Clang, and newer Go.
1.1.0 already runs on Trusty without issues.

To do this, we need to disable afalgeng in the -Werror build.

afalgeng isn't compatible with the -Werror build on
Travis Trusty due to kernel header mismatch.

(See also 97043e46

)

Reviewed-by: Richard Levitte <levitte@openssl.org>