Filter 'vendor' directory from coverage results

A common pattern in Ruby development is to install the development
dependencies under the 'vendor/bundle' subdirectory of the project
directory. These end up in the coverage results.

Fix this by explicitly filtering the 'vendor' directory from the
coverage results.
Remove hashbang line from Rakefile

The file is not executable and even if it was, in practice it would
still need to be executed using 'bundle exec'.
Remove 'executables' from Gem specification

There are none.
Remove 'require_paths' from Gem specification

This is set by default.
Simplify 'test_files' in Gem specification

All tests are in the 'spec' directory.
Merge pull request #11 from jvirtanen/cleanups/gemspec

Clean up Gem specification
Merge pull request #10 from jvirtanen/cleanups/rakefile-hashbang

Remove hashbang line from Rakefile
Merge pull request #9 from jvirtanen/cleanups/coverage-filter

Filter 'vendor' directory from coverage results