Turn off Coverage tests under JRuby.

 see https://github.com/metricfu/metric_fu/pull/226
     https://github.com/jruby/jruby/issues/1196
     https://jira.codehaus.org/browse/JRUBY-6106
     https://github.com/colszowka/simplecov/issues/86

Travis ENV modified per
http://docs.travis-ci.com/user/ci-environment/#Environment-variables

@enebo https://twitter.com/tom_enebo/status/462289024107814914
recommended JRUBY_OPTS="-Xcli.debug=true"
but that didn't work and caused other failures
JRUBY_OPTS="--debug" Still produced the below error, as did
JRUBY_OPTS="--debug -Xcli.debug=true"

Failure message was
home/travis/build/metricfu/metric_fu/gemfiles/vendor/bundle/jruby/1.9/gems/simplecov-0.8.2/lib/simplecov.rb:31 warning: tracing (e.g. set_trace_func) will not capture all events without --debug flag

SimpleCov::Formatter::MetricFu calculates the same coverage from an RCov report as from SimpleCov
     Failure/Error: expect(source_file.coverage.count).to eq(covered_lines_from_rcov_text.count)

       expected: 11
            got: 8

       (compared using ==)
     # /home/travis/build/metricfu/metric_fu/spec/metric_fu/metrics/rcov/simplecov_formatter_spec.rb:52:in `(root)'
     # /home/travis/build/metricfu/metric_fu/spec/support/timeout.rb:6:in `(root)'