minimum_coverage_by_file now prints the name of the violating file.

[philipritchey]

when minimum coverage by file is set, output used to (currently) look like:

Line coverage by file (83.33%) is below the expected minimum coverage (90.00%).
Branch coverage by file (77.77%) is below the expected minimum coverage (90.00%).

now, with in <filename> at the end of the line, it (could) looks like:

Line coverage by file (83.33%) is below the expected minimum coverage (90.00%) in foo.rb.
Branch coverage by file (77.77%) is below the expected minimum coverage (90.00%) in foo.rb.