Fix submodule init on clone

In a previous pull request, `clone` was changed to initialize and update submodules recursively.  This change used a flag, `--recurse-submodules`, that was added after the version of git being used by the dea.  This flag was actually an alias to `--recursive` which does exist in the version of git used by the dea[1].

The fix made for the previous failure was to split the `clone` command into two calls (`clone`, then change directory and initialize submodules) which was unnecessary.  In addition, this fix added some logic around cloning with `--depth=1` that broke this depth functionality when cloning with `http://` URLs (the default when using GitHub).

This change has the net effect of reverting the changes in 20bcb31275e1605d626fb1869dc9fb8314252dd0 and doing the simplest possible fix for the `--recurse-submodules` change.  Specifically this change rolls back the changes around `--depth=1` as they are not necessary to fix the problem with the unsupported flag and break the functionality that `--depth=1` was intended to provide.

[1] The options clone supports in the version of Git on the dea:
```plain
-----> Downloaded app package (23M)
-----> Downloaded app buildpack cache (4.0K)
error: unknown option `recurse-submodules'
usage: git clone [options] [--] <repo> [<dir>]

    -q, --quiet           be quiet
    -v, --verbose         be verbose
    --progress            force progress reporting
    -n, --no-checkout     don't create a checkout
    --bare                create a bare repository
    --mirror              create a mirror repository (implies bare)
    -l, --local           to clone from a local repository
    --no-hardlinks        don't use local hardlinks, always copy
    -s, --shared          setup as shared repository
    --recursive           initialize submodules in the clone
    --template <path>     path the template repository
    --reference <repo>    reference repository
    -o, --origin <branch>
                          use <branch> instead of 'origin' to track upstream
    -b, --branch <branch>
                          checkout <branch> instead of the remote's HEAD
    -u, --upload-pack <path>
                          path to git-upload-pack on the remote
    --depth <depth>       create a shallow clone of that depth

/var/vcap/packages/dea_next/buildpacks/lib/buildpack.rb:33:in `clone_buildpack': Failed to git clone buildpack (RuntimeError)
	from /var/vcap/packages/dea_next/buildpacks/lib/buildpack.rb:41:in `build_pack'
	from /var/vcap/packages/dea_next/buildpacks/lib/buildpack.rb:26:in `block in compile_with_timeout'
	from /usr/lib/ruby/1.9.1/timeout.rb:68:in `timeout'
	from /var/vcap/packages/dea_next/buildpacks/lib/buildpack.rb:25:in `compile_with_timeout'
	from /var/vcap/packages/dea_next/buildpacks/lib/buildpack.rb:16:in `block in stage_application'
	from /var/vcap/packages/dea_next/buildpacks/lib/buildpack.rb:12:in `chdir'
	from /var/vcap/packages/dea_next/buildpacks/lib/buildpack.rb:12:in `stage_application'
	from /var/vcap/packages/dea_next/buildpacks/bin/run:10:in `<main>'
```