Add solargraph.yml file as root path search file.

Solargraph allows to set configuration options by creating a
.solargraph.yml file at the root of the project using it. Therfore this
file is a good canditate for finding ruby projects root paths.

Initial discussion:
  https://github.com/w0rp/ale/issues/1874#issuecomment-418316168

autoload/ale/ruby.vim

@@ -29,7 +29,7 @@ function! ale#ruby#FindProjectRoot(buffer) abort
       return l:dir
     endif
 
-    for l:name in ['Rakefile', 'Gemfile']
+    for l:name in ['.solargraph.yml', 'Rakefile', 'Gemfile']
         let l:dir = fnamemodify(
         \   ale#path#FindNearestFile(a:buffer, l:name),
         \   ':h'

test/command_callback/test_ruby_solargraph.vader

@@ -27,3 +27,10 @@ Execute(should set solargraph for ruby app2):
   AssertLSPOptions {}
   AssertLSPProject ale#path#Simplify(g:dir . 'command_callback/../ruby_fixtures/valid_ruby_app2')
   AssertLSPAddress '127.0.0.1:7658'
+
+Execute(should set solargraph for ruby app3):
+  call ale#test#SetFilename('../ruby_fixtures/valid_ruby_app3/lib/file.rb')
+  AssertLSPLanguage 'ruby'
+  AssertLSPOptions {}
+  AssertLSPProject ale#path#Simplify(g:dir . 'command_callback/../ruby_fixtures/valid_ruby_app3')
+  AssertLSPAddress '127.0.0.1:7658'