- fixed return code of 1 when updating from git

[jarrettchisholm]

When used with a privisioning system that monitors exit status, updating from git fails on line 72:
cd "$NVM_DIR" && command git checkout --quiet $(nvm_latest_version) && command git branch --quiet -D master >/dev/null 2>&1

This is because master doesn't exist.

This patch checks for the existance of master before deleting the branch.

[ljharb]

Good lord, this is a great catch. This might fix #559, #685, and #649.

As soon as the tests pass, I'll merge this.