Fixes a bug in Stringends_with?

  iex> String.ends_with? "hello", ["hell", "lo"]
  false

 The problem was that the implementation uses :binary.matches, and that returns only
 one match. It would find "hell", but pos+len != size, and so returned false.

 Changed for a less efficient implementation.
Switch to using pure binaries
Merge pull request #1117 from pragdave/master

Fix for a problem with String.ends_with?