Ignore /.cask
Use alias.
Ecukes depends on Espuds.
Use cl-lib for assert.
Avoid backreference inside a character class (fixes #4)

The author of this regexp is actually @sergeypererva. Kudos, man!

The problem with the original regexp is that it uses something that a
regexp shouldn't normally contain: [^\1] is not a valid thing.

More information: http://www.regular-expressions.info/backref.html
(See section "Parentheses and Backreferences Cannot Be Used Inside
Character Classes")
features/string-conversion.feature: add a failing test

Although the new regexp makes all the tests pass, it doesn't cover the
case, which was previously covered by the original regexp. This test
proves the statement. If you run the test suit with the old regexp, it
should still contain only one, the same old, failure: "Convert string
with the number 1 in it". However this test will pass. On the contrary,
the new regexp can't please this test.