Fixed `nvm_version_greater` has syntax error in zsh-buildin-command `[`

The following error now occurs when you load a nvm.sh in zsh-5.0.5. 

	nvm_version_greater: 5: condition expected:> 

Cause Because zsh-built-in `[` can not understand the operator "\>".
I have to fix the above error so that it does not appear to fix the -gt operators in this pull-reqest.