Handle unset $HOME

This happens when invoking `r10k deploy environment -p` through
a prerun_command in puppet.conf.

Part of the error message:

Debug: Executing 'r10k deploy environment -p'
Error: Could not run command from prerun_command: Execution of
'r10k deploy environment -p' returned 1:
/usr/lib64/ruby/gems/1.9.1/gems/r10k-1.1.0/lib/r10k/git/cache.rb:16:in
`expand_path': couldn't find HOME environment -- expanding `~' (ArgumentError)
Merge pull request #70 from tampakrap/argumenterror

Handle unset $HOME