Prism inadvertently locked to ~0.15

[j15e]

I think that the gemspec was inadvertently modified to ~> 0.15" to test for older versions support in #71, but this was actually committed and now this gem locks prism to an old version.

Is that correct?

On my end this causes problem with ruby-lsp because newer versions require a new version of prism and bundler resolve ruby-lsp to a quite old version instead of using the latest version we want.