Fix cache dir issue - #13

After triaging #13, there were two issues were found which affects folks who have not defined a cacheDir (default dir where the code you intend to deploy to gh-pages is cached -- to avoid frequent git clones)

1. if cacheDir is not set, copying files from src dir to a destination dir fails. Refer Line 107 in index.js. Reverted to using the repo dir. This will ensure there is always a dir to copy files to i.e either user set directory or default tmp directory.

2. `if (fs.existsSync(repo.dot_git))` --> This always seems to be true because an object of gift - the git client lib always has `obj.dot_git` set. So I reverted to verifying whether there was an existing .git folder in the directory