If running `bin/rails t test/models/bunny_test.rb:` we'd implicitly run all the
tests in the bunny test. I now highly doubt that people would ever put in a line
filter without a line *and* want that to mean run all tests in that file.

Instead, change regex to require a line digit after the colon, so runs without a
line at the end would fail to require the test file.

This also has the side benefit of breaking requiring a file with many colons:
`bin/rails t test/models/bunny_test.rb:::::::::::::4`

Think this means I've had enough colonoscopy to last me through the year :)