Re-order and group test helpers
Don't allow tests to fail on Emacs snapshot

MELPA is being fixed for Emacs snapshot
Use built-in ERT tools to skip tests
Allow for Emacs 24 failures on Travis CI
Stub test skipping
Stub test skipping on Emacs 24.3

If ERT cannot skip tests, we fake the corresponding functions to signal
a custom error, and mark all tests as expected to fail on this error.

Not nice, but works :)
Simplify GPG check
Remove build status from README

Doesn't provide much value, and the build is failing since a long
time anyway
Skip selection test if checkers are missing
Merge branch 'refactor-test-skipping'

* refactor-test-skipping:
  Skip selection test if checkers are missing
  Remove build status from README
  Simplify GPG check
  Stub test skipping on Emacs 24.3
  Stub test skipping
  Allow for Emacs 24 failures on Travis CI
  Use built-in ERT tools to skip tests