Include lib/rdoc/generator/pot/* in built gem

[tjkirch]

This fixes #391, the fact that the lib/rdoc/generator/pot/* files are missing in the published 4.2.1 gem.

Testing done

Before:

$ git checkout master && rm -r pkg \
   && rake package \
   && find pkg/rdoc-4.2.1 -path '*pot*' \! -path '*/test/*'
...
pkg/rdoc-4.2.1/lib/rdoc/generator/pot.rb

After:

$ git checkout missing_pot && rm -r pkg \
   && rake package \
   && find pkg/rdoc-4.2.1 -path '*pot*' \! -path '*/test/*'
...
pkg/rdoc-4.2.1/lib/rdoc/generator/pot.rb
pkg/rdoc-4.2.1/lib/rdoc/generator/pot
pkg/rdoc-4.2.1/lib/rdoc/generator/pot/message_extractor.rb
pkg/rdoc-4.2.1/lib/rdoc/generator/pot/po_entry.rb
pkg/rdoc-4.2.1/lib/rdoc/generator/pot/po.rb

[tjkirch]

The Travis failure was a benchmark on Ruby 2.1:

TestRDocContext#bench_add_include [/home/travis/build/rdoc/rdoc/test/test_rdoc_context.rb:419]:

Expected 0.8893409014024847 to be >= 0.9.

I've found this same benchmark to be flaky, before and after my change, and I was using Ruby 2.2.

[zzak]

@tjkirch Thank you!