Fix node version detection in nvm_install_latest_npm #324

Job	Run time
tests (install_script, bash)	1m 17s
tests (sourcing, sh)	43s
tests (sourcing, bash)	47s
tests (sourcing, dash)	47s
tests (sourcing, zsh)	1m 5s
tests (slow, sh)	3m 23s
tests (slow, bash)	3m 21s
tests (slow, dash)	3m 7s
tests (slow, zsh)	3m 45s
tests (installation_iojs, sh)	1m 0s
tests (installation_iojs, bash)	1m 1s
tests (installation_iojs, dash)	1m 5s
tests (installation_iojs, zsh)	1m 20s
all test suites, all shells	1s
	22m 42s

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