[PR 245] Ensure we return Floats, not Fixnum or Rational; fixes segfaults with mathn

[Benjamin Fleischer]

Basically, mathn can cause a segfault when calling round on a Rational

https://github.com/colszowka/simplecov/pull/245

Fixes include  [#175][#140][#266]

Fix is already in the wild in https://github.com/codeclimate/ruby-test-reporter/pull/15

Underlying bug in MRI has been fixed in more recent releases of CRuby 2.0 and 1.9.3 https://bugs.ruby-lang.org/issues/8687 and https://bugs.ruby-lang.org/issues/8924

-Benjamin