Add `author` to generated gemspec

This commit fixes the warning

  fixture at bump/spec/fixture did not have a valid gemspec.
  This prevents bundler from installing bins or native extensions, but
  that may not affect its functionality.
  The validation message from Rubygems was:
    authors may not be empty
  Using fixture (1.0.0) from source at .
  Using bundler (1.5.3)