`global` command removes "perl-" prefix

`install` command removes "perl-" prefix but `global` doesn't.

ex)
This works:

  $ plenv install perl-5.17.8
  #=> Install 5.17.8 to /Users/foo/.plenv/versions/5.17.8

But this doesn't work:

  $ plenv global perl-5.17.8
  #=> perl-5.17.8 is not installed on plenv. at /usr/local/bin/plenv line 189.
Merge pull request #2 from a2ikm/global_remove_prefix

`global` command removes "perl-" prefix