Closes #105: Port indentation tests to use ERT

* contrib/test/slime-cl-indent-test.txt: Moved to from previous file and
changed test names from numbers to symbols.

* contrib/test/slime-indentation-tests.el: New file. Defines ERT
indentation tests from specs in `slime-cl-indent-test.txt'.

* contrib/slime-cl-indent.el: Moved tests to
test/slime-indentation-tests.el. In file comments, explain the
legacy cl-indent.el situation. Provide `slime-cl-indent'.

* contrib/slime-indentation.el (slime-cl-indent): `require' instead of
`load'.
Check slime-indentation contrib in Travis CI

* .travis.yml: Check slime-indentation contrib in travis, but allow
failures for now. Notice that tests pass for now (Emacs 24.3), but at
least 5 will fail on upcoming Emacs 24.4.