Support Ruby 2.7.0

[raxod502]

Similar to ruby-syntax-tree/syntax_tree#144, this package is also a dependency of running Prettier for Ruby code on Ubuntu 20.04.

Since @kddnewton was kind enough to add Ruby 2.7.0 support to syntax_tree proper in ruby-syntax-tree/syntax_tree#148, I figured I would try to update syntax_tree-rbs myself. Unfortunately, having little experience with Ruby, my work has proceeded very slowly. I get these errors when running the tests under 2.7.0:

186) Error:
RBSTest#test_class:
FrozenError: can't modify frozen String: "class Foo\nend\n"
    /home/raxod502/files/code/lib/syntax_tree-rbs/.bundle/vendor/ruby/2.7.0/gems/prettier_print-0.1.0/lib/prettier_print.rb:274:in `<<'
    /home/raxod502/files/code/lib/syntax_tree-rbs/.bundle/vendor/ruby/2.7.0/gems/prettier_print-0.1.0/lib/prettier_print.rb:761:in `block in flush'
    /home/raxod502/files/code/lib/syntax_tree-rbs/.bundle/vendor/ruby/2.7.0/gems/prettier_print-0.1.0/lib/prettier_print.rb:761:in `each'
    /home/raxod502/files/code/lib/syntax_tree-rbs/.bundle/vendor/ruby/2.7.0/gems/prettier_print-0.1.0/lib/prettier_print.rb:761:in `flush'
    /home/raxod502/files/code/lib/syntax_tree-rbs/lib/syntax_tree/rbs.rb:51:in `format'
    /home/raxod502/files/code/lib/syntax_tree-rbs/test/rbs_test.rb:343:in `assert_format'
    /home/raxod502/files/code/lib/syntax_tree-rbs/test/rbs_test.rb:163:in `test_class'

187) Error:
RBSTest#test_interface_5:
TypeError: no implicit conversion of Integer into Array
    /home/raxod502/files/code/lib/syntax_tree-rbs/.bundle/vendor/ruby/2.7.0/gems/prettier_print-0.1.0/lib/prettier_print.rb:782:in `-'
    /home/raxod502/files/code/lib/syntax_tree-rbs/.bundle/vendor/ruby/2.7.0/gems/prettier_print-0.1.0/lib/prettier_print.rb:782:in `flush'
    /home/raxod502/files/code/lib/syntax_tree-rbs/lib/syntax_tree/rbs.rb:51:in `format'
    /home/raxod502/files/code/lib/syntax_tree-rbs/test/rbs_test.rb:343:in `assert_format'
    /home/raxod502/files/code/lib/syntax_tree-rbs/test/rbs_test.rb:16:in `block (2 levels) in test_fixtures'

For some reason, the pretty-printing library seems like it's trying to modify one of the string literals that gets passed in from the test fixtures in syntax_tree-rbs, and I haven't been able to figure out why that's happening yet. The tests for prettier_print pass under 2.7.0, so I figure it isn't a problem within that library (or at least it's a bug that isn't known yet).

I'll continue working on this when I have time, but if anyone more familiar with Ruby wants to take a look, it'd probably speed things up.

[raxod502]

Whoopsie, I was just being silly. Once I fixed my first change, the errors I mentioned above went away. I'll see about updating documentation and build files.

[raxod502]

It looks like the CI here is configured to run using the workflow from main, but I ran the Rake tests locally using Ruby 2.7.0 and they passed. I also installed this branch into my Ubuntu 20.04 machine and confirmed that Prettier now works for Ruby code. I think this should be ready for review!

[kddnewton]

Yeah looks good!

[raxod502]

Mind tagging this as 0.5.1 so the new version can be installed by default?

[kddnewton]

Yeah this is released!