Change double quotes to single quotes in README

The install command in the README throws out this error.

    Bundler, the underlying system Vagrant uses to install plugins,
    reported an error. The error is shown below. These errors are usually
    caused by misconfigured plugin installations or transient network
    issues. The error from Bundler is:

    Net::HTTPNotFound: No gems found matching "vagrant-vmware-workstation" "2.3.6" nil

I discovered that using single quotes instead of double quotes resolved the issue for me.