PROTON-755: Require specific version of minitest for Ruby tests.

The require for the smoke tests now require a version between 4.7.0 and
not 5.0.
PROTON-755: Updated rspec tests to use the newer expect syntax

Rspec is moving away from monkey patching objects, and this new syntax
reflects that change. Updated the rspec tests to work with Rspec 2.4 and
3.x properly.
PROTON-755: Add SecureRandom for use by Ruby 1.8 tests
PROTON-755: Changed how the Data rspec test sets a single random character.

The new method is portable across versions of Ruby.
NO-JIRA: Tweaked how Ruby 1.8 determines if a string is UTF-8

Uses the kconv stdlib module to add the "isutf8" method.