Check libraries to make sure they meet Ripple's minimum requirements.

* Ripple depends on two libraries, boost and OpenSSH.  A new function
  version::checkLibraryVersions() returns an error string if either of
  these libraries is too old, or an empty string otherwise.

* The call to version::checkLibraryVersions() during rippled startup
  will be added in a later change.

* With unit tests.