Add SimpleCov and Coveralls.io dependenices
Add rake tasks for travis, which include publishing coverage reports.
Change travis to use a rake task instead of shindont directly.
This is partly necessary for
https://github.com/lemurheavy/coveralls-ruby/pull/20, though
I'm a fan of running just "rake" or "rake travis" for simplicity
anyways.
The actual simplecov/coveralls setup.
The use of Process.pid is an attempt to avoid confusing SimpleCov when
running tests in parallel.  Other tests frameworks take a similar
approach when they detect parallel_tests.
Merge pull request #1897 from maxlinc/code_coverage

Code coverage