Fix bug when trying to select already selected version.
Fix bug when trying to select already selected version.

Steps to reproduce bug (without this patch):

1 - nvm use 0.10.32
2 - .node-version with 0.10.32 (same as already selected version)
3 - cd to directory containing above .node-version
4 - error returned: avn could not activate node 0.10.32

wbyoung/avn-nvm,wbyoung/avn-nvm,kuy/avn-nvm,kuy/avn-nvm