Let -v respond with gem version instead of 'unknown'

This is what was printed before:
```
➜  dead_end git:(main) ✗ dead_end --version
dead_end: version unknown
```

Now we get the actual gem version in the output.

Author: mauro-oto

CHANGELOG.md

@@ -1,5 +1,7 @@
 ## HEAD (unreleased)
 
+- Let -v respond with gem version instead of 'unknown' (https://github.com/zombocom/dead_end/pull/82)
+
 ## 2.0.0
 
 - Support "endless" oneline method definitions for Ruby 3+ (https://github.com/zombocom/dead_end/pull/80)

exe/dead_end

@@ -35,6 +35,8 @@ parser = OptionParser.new do |opts|
     Options:
   EOM
 
+  opts.version = DeadEnd::VERSION
+
   opts.on("--help", "Help - displays this message") do |v|
     puts opts
     exit

spec/integration/exe_cli_spec.rb

@@ -62,5 +62,10 @@ def exe(cmd)
         expect(tmp_dir).to_not be_empty
       end
     end
+
+    it "prints the version" do
+      out = exe("-v")
+      expect(out.strip).to include(DeadEnd::VERSION)
+    end
   end
 end