Add '--depth' to 'git submodule update'

Fixes https://github.com/travis-ci/travis-ci/issues/1867
Merge branch 'master' into ha-feature-gh-submodule-depth

Conflicts:
	spec/shared/git.rb
Make --depth flag optional for 'git submodule update'

The opts path `git.submodules_depth` is up for debate
Define depth_opt in all situations
Announce the source of env vars
Use ssh key from settings
Updating some golang defaults

to set 1.3 as the default version and to use the new gvm binary download
if possible.
Using the new gvm --preferbinary flag
It's better to ensure vars is an array in initializer than in if checks
Simplify ssh_key installing by using encoded attribute
Announce env vars from config as env vars from .travis.yml
Typo
Deployment: Use build data rather than environmental variable to check whether build is a pull request
Merge pull request #268 from Joshua-Anderson/deployment-patch

Deployment: Use build data rather than environmental variable to check whether build is a pull request
Merge pull request #259 from meatballhat/go-version-update

Updating some golang defaults
Add newline before env vars announcements
Make source key installing backwards compatible
hard code getcomposer.org ip address
fix gvm flag
Reverting to gvm install fallback behavior without `--prefer-binary`