Fix installation paths in gemspec

There is no need to explicitly add "ext" to the require_paths; RubyGems
will automatically copy the built bundle over to the lib during
installation:

http://guides.rubygems.org/specification-reference/#require_paths

Use "ruby-watchman" as a prefix in the extension configuration to ensure
that it gets installed in a subdirectory of lib; thus we can continue to
use a "ruby-watchman/ext" as a require target and not have to come up
with something globally unique.

Bumped the version number so that I could test this independently with:

    bundle exec rake # compiles, tests
    bundle exec rake build # makes new gem
    sudo gem install ./ruby-watchman-0.0.2.gem # installs
    irb -rrubygems -rruby-watchman # test using example from README