Remove use of deprecated 'should_not raise_error(SpecificError)'

RSpec has since deprecated this construct. The recommended replacement

  @test.should_not raise_error

works just as well for us.

See: rspec/rspec-expectations#231
Add root .rspec config file to auto-require turnip

Took me a while to figure out that RSpec needed the '-r turnip/rspec'
option. Specifying it in the project RSpec config should save everyone
else the trouble.
Merge pull request #6 from benesch/fix-tests

Fix tests