Fixes a minor compatibility issue with the latest release of testthat

Breaking Changes

Documentation syntax changes. Documentation will be need to be
re-roxygenize'd or otherwise updated.

• Changes syntax of tests to minimize reliance on testex namespace
  consistency across versions. Instead of using testex(with_srcref(..)) and
  testthat_block(test_that(.., with_srcref(..))), both interfaces are now
  handled via testex() with an added style parameter:

  testex(style = "testthat", srcref = "fn.R:10:11", { code })

  This syntax is intended to be more resilient to changes to keep your
  tests from relying too heavily on an unchanging testex function interface.

New Features

• Adds configuration (Config/testex/options) field "version", which is
  automatically updated when a newer version of testex is first used.

  This field is checked to decide whether the currently loaded version of
  testex is capable of re-running your tests.

  Currently, a conservative approach is taken. If there is a version mismatch,
  testex will suggest updating when run independently using a testing
  framework and will disable testex testing during R CMD check to avoid
  causing downstream test failures as the API changes. However, this means
  that testex tests will be ineffective if your package is out-of-date
  with the released testex version on CRAN

  Past a v1.0.0 release, this behavior will be relaxed to check for a
  compatible major version.

Full Changelog: v0.1.0...v0.2.0

Initial CRAN release 🎉