Make sure sourcing nvm.sh on shells that don't support source options doesn't exit nonzero.

Fixes #721

test/sourcing/Sourcing nvm.sh with no default should return 0

@@ -0,0 +1,11 @@
+#!/bin/sh
+
+die () { echo $@ ; exit 1; }
+
+touch ../../alias/default
+rm ../../alias/default || die 'removal of default alias failed'
+nvm_alias default && die '"nvm_alias default" did not fail'
+
+set -e # necessary to fail internally with a nonzero code
+
+. ../../nvm.sh || die 'sourcing returned nonzero exit code'