New call to use_ok messes up Test::CPAN::Changes plan

I noticed the new version of the plugin calls `use_ok()`, which caused my module to do the following. The attached patch fixes this issue by running `changes_ok()` as a subtest.

    $ RELEASE_TESTING=1 prove --lib t/release-cpan-changes.t
    t/release-cpan-changes.t .. 1/?
    #   Failed test 'planned to run 4 but done_testing() expects 5'
    #   at /home/mgardner/perl5/perlbrew/perls/perl-5.16.3/lib/5.16.3/Test/More.pm line 221.
    # Looks like you planned 4 tests but ran 6.
    # Looks like you failed 1 test of 6 run.
    t/release-cpan-changes.t .. Dubious, test returned 1 (wstat 256, 0x100)
    All 4 subtests passed
    
    Test Summary Report
    -------------------
    t/release-cpan-changes.t (Wstat: 256 Tests: 6 Failed: 2)
      Failed tests:  5-6
      Non-zero exit status: 1
      Parse errors: Plan (1..4) must be at the beginning or end of the TAP output
                    Bad plan.  You planned 4 tests but ran 6.
    Files=1, Tests=6,  0 wallclock secs ( 0.03 usr  0.00 sys +  0.02 cusr  0.00 csys =  0.05 CPU)
    Result: FAIL