README should use 'new_version.activate!', not 'new_version.activate'

The correct method is `activate!` (presumably because it is destructive) - see [version.rb L81-L83](https://github.com/fastly/fastly-ruby/blob/master/lib/fastly/version.rb#L81-L83)
Merge pull request #13 from samjsharpe/master

README should use 'new_version.activate!', not 'new_version.activate'