Change plugin name to correspond gem name

Use the same name so that `Vagrant.has_plugin?` can be used with same
argument as `vagrant plugin install`.

Fixes #47
Merge pull request #52 from tmatilai/plugin-name

Change plugin name to correspond gem name
Fix installed version check

Current implementation does not correctly get a version of the installed Chef
on some platforms, such as CentOS 5.

Problem is that sudo result contains the blank line at the last,
and version is overwritten by the blank.