Need to un-require and then re-require Docile after SimpleCov is
loaded, so that it can measure test coverage of the library. This
is necessary since SimpleCov began to ship with Docile as a
dependency.
Weirdly, on coveralls.io, only Rubinius (rbx-19mode) shows a 1%
lack in coverage due to a line in docile_spec.rb. This should
ensure that the spec files are never measured for code coverage.