Fix module install from forge fails install when modules directory is not yet created

r10k version: 1.0.0rc3 d7fc40007713c1b7a8bd4df8eac6030a01a50212

Running "r10k deploy environment -p" on a new environment fails for forge modules.

Steps to reproduce
1. Create new environment
2. Create Puppetfile with both forge modules and git modules.
3. Run "r10k deploy environment -p".

What happens:

Forge modules fail to install. Git modules are installed. A second run of r10k installs the forge modules without error. The "git clone" command automatically creates parent directories.

Using some extra debug output, "puppet module install" prints

    "Error: Could not install module 'adrien-pe_upgrade' (latest)\n  Directory /etc/puppetlabs/puppet/environments/master/modules does not exist"

What's expected:

r10k will create the modules directory prior to installing the forge modules.