Cache rendered template in tests.
List development dependencies as BuildRequires.

The BuildRequires are kept commented out, since they are typically
needed just in case the test suite is running. Also, not all development
dependencies are always needed anyway.

The Rake and Bundler are never listed among BR, since we discourage
their usage.

The RDoc is filtered out as well, since RDoc is pulled in anyway via
RubyGems dependencies.