Fixed #67.

* bootstrap: Changed to dereference `latest` value of `--with-binutils`
  command-line option into a concrete version number before invoking `bjam`.
* jamroot: Removed support for `latest` value of `--with-binutils`
  command-line option.
* binutils/download.sh: Removed support for `latest` value of the
  command-line argument.
Merge pull request #68 from Cryolite/fix-67

Fixed #67.