Add solargraph language server

Author: erikw

Gemfile

@@ -12,10 +12,11 @@ gemspec
 # Reference: https://github.com/rubygems/bundler/pull/7222
 # However there's an argument for using gemspec too: https://bundler.io/guides/creating_gem.html#testing-our-gem
 group :development, :test do
-  gem "rake", "~> 13.0", require: false
-  gem "travis", "~> 1.0", require: false
   gem "appraisal", "~> 2.4", require: false
   gem "gem-release", "~> 2.0", require: false
+  gem "rake", "~> 13.0", require: false
+  gem "solargraph", require: false
+  gem "travis", "~> 1.0", require: false
 end
 
 group :test do