Update Ruby gem version to 3.0.0.alpha.2.0.

This update conforms to our two-numbers-after-alpha scheme that allows
us to bump the last number if we need to re-upload a gem. (Rubygems does
not allow re-use of a version number once a gem is uploaded.)

Change-Id: Ia8e7c129d19800afd66f8052785cf5a00462c7ba