We often need to clear cache.

For example, in web application development, when we changed source code, we would like the change applied to the app server without reboot.

In the case of a Rails application, this requires the following configuration:

```ruby
Rails.application.configure do
  config.to_prepare do
    MethodSource.instance_variable_set(:@lines_for_file, {})
  end
end
```

It seems so ugly, isn't it? So I feel we need to add `MethodSource.clear_cache` interface.

```diff
  Rails.application.configure do
    config.to_prepare do
-     MethodSource.instance_variable_set(:@lines_for_file, {})
+     MethodSource.clear_cache
    end
  end
```

The error message for `issue = %W/` changed, before:

```
(eval):2: syntax error, unexpected tSTRING_END, expecting tSTRING_CONTENT or tSTRING_DBEG or tSTRING_DVAR or ' '
```

After:

```
((eval):2: unterminated list meets end of file)
```

Fixes #52 (Revert the JRuby 9.2.0.0 source_location hack once the newest version
is out)

This reverts commit 20fb1efde4530c78f7627d8b5b4e0b3f43cf821d.

Fixes https://github.com/pry/pry/issues/1804
(JRuby 9.2.0.0 breaks `source_location` and therefore our test suite)

JRuby 9.2.0.0 fails to fetch source code for procs because of the bug:
https://github.com/jruby/jruby/pull/5262

The problem is that source_location is reported at the end of the proc, instead
of the beginning.

The way I fix it is rather dumb (rewinding back and checking if it's complete
expression) but it's isolated only to 9.2.0.0 and likely won't be needed when
another JRuby is released. However, so far it's the latest release.

ruby 2.4 support