Updating some golang defaults

to set 1.3 as the default version and to use the new gvm binary download if possible.
I think you're correct, but I'm not so sure that it actually works:
```console
$ docker run -it --rm quay.io/travisci/travis:go su - travis
travis@0e6dbe1c15c4:~$ gvm get
Your branch is up-to-date with 'origin/master'.
remote: Counting objects: 37, done.
remote: Compressing objects: 100% (28/28), done.
remote: Total 37 (delta 17), reused 22 (delta 8)
Unpacking objects: 100% (37/37), done.
From https://github.com/moovweb/gvm
   99876c1..b914f5a  master     -> origin/master
Updating 99876c1..b914f5a
Fast-forward
 README.md                                          | 10 +--
 scripts/env/pkgset-use                             |  2 +-
 scripts/install                                    | 92 +++++++++++++---------
 .../gvm_install_1.2_preferbinary_comment_test.sh   |  1 +
 4 files changed, 60 insertions(+), 45 deletions(-)
 create mode 100644 tests/scenario/gvm_install_1.2_preferbinary_comment_test.sh
travis@0e6dbe1c15c4:~$ gvm install go1.3 --preferbinary
WARNING: Invalid option --preferbinary
Usage: gvm install [version] [options]
    -s,  --source=SOURCE      Install Go from specified source.
    -n,  --name=NAME          Override the default name for this version.
    -pb, --with-protobuf      Install Go protocol buffers.
    -b,  --with-build-tools   Install package build tools.
    -B,  --binary             Only install from binary.
    -h,  --help               Display this message.
travis@0e6dbe1c15c4:~$ gvm install go1.3 --prefer-binary
Installing go1.3 from binary source
ERROR: Failed to download binary go from http://golang.org. Trying https://go.googlecode.com
ERROR: Failed to download binary go
```