Travis: Skip gcc on Linux with default settings

    ## gcc on OSX is less useful, because the default compiler is clang.

    - compiler: gcc

      os: osx

    ## gcc on Linux with no env is redundant, because all the custom builds use

    ## gcc on Linux

    - compiler: gcc

      os: linux

      env:

## We don't need sudo. (The "apt:" stanza after this allows us to not need

## sudo; otherwise, we would need it for getting dependencies.)

  o Minor features (continuous integration):

    - Skip gcc on OSX in Travis CI, it's rarely used.

      Skip a duplicate hardening-off build in Travis on Tor 0.2.9.

      Skip gcc on Linux with default settings, because all the non-default

      builds use gcc on Linux.

      Implements ticket 27252.