Ensure that a bumped version is used for `tag` and `release`.

The gemspec file was not being properly re-evaluated after the version file changed
 in rubygems 2.1.11, which led to the old version being used to tag and release
 gems that were just bumped.

 This workaround ensures that the new version number will be used by subsequent
 commands by passing it as an option that will be evaluated prior to the gemspec
 version.

[Fixes #36]