Remove Ruby warning

`lib/simplecov/jruby16_fix.rb:31: warning: assigned but unused variable - e`

The previous catch-all `rescue` was used to swallow this exception:
`Errno::ENOENT: No such file or directory - test/fixtures/generated_buddha.rb`
We now make sure the file is present with `File.file?`.

Also reverts 6f711098dbe3dccac067f1a128fc33ef2f845079 and moves the fix
into jruby_fix.rb to keep JRuby fixes in the same file.
Remove Ruby warning

`lib/simplecov/defaults:57: warning: instance variable @exit_status not initialized`

We now ensure that @exit_status is always initialized.

Also did some cosmetic arrangements with no functional changes.
Merge #213, use jruby fix only on < 1.7