*/diagnostic: tweak messaging.

Update some messaging based on recent user confusion and the
`/usr/local/Homebrew` and Ruby 2 migrations.

Library/Homebrew/diagnostic.rb

@@ -389,10 +389,10 @@ module Homebrew
         return if HOMEBREW_PREFIX.to_s == "/usr/local"
 
         <<-EOS.undent
-          Your Homebrew is not installed to /usr/local
-          You can install Homebrew anywhere you want but some bottles (binary
-          packages) can only be used in /usr/local and some formulae (packages)
-          may not build correctly unless you install in /usr/local. Sorry!
+          Your Homebrew's prefix is not /usr/local.
+          You can install Homebrew anywhere you want but some bottles (binary packages)
+          can only be used with a /usr/local prefix and some formulae (packages)
+          may not build correctly with a non-/usr/local prefix.
         EOS
       end
 

Library/Homebrew/extend/os/mac/diagnostic.rb

@@ -65,8 +65,8 @@ module Homebrew
         return unless MacOS::Xcode.installed? && MacOS::Xcode.outdated?
 
         message = <<-EOS.undent
-          Your Xcode (#{MacOS::Xcode.version}) is outdated
-          Please update to Xcode #{MacOS::Xcode.latest_version}.
+          Your Xcode (#{MacOS::Xcode.version}) is outdated.
+          Please update to Xcode #{MacOS::Xcode.latest_version} (or delete it).
           #{MacOS::Xcode.update_instructions}
         EOS
 
@@ -171,8 +171,7 @@ module Homebrew
       end
 
       def check_ruby_version
-        ruby_version = MacOS.version >= "10.9" ? "2.0" : "1.8"
-        return if RUBY_VERSION[/\d\.\d/] == ruby_version
+        return if RUBY_VERSION[/\d\.\d/] == "2.0"
 
         <<-EOS.undent
           Ruby version #{RUBY_VERSION} is unsupported on #{MacOS.version}. Homebrew
@@ -264,8 +263,8 @@ module Homebrew
         return if installed_version >= latest_version
 
         <<-EOS.undent
-          Your XQuartz (#{installed_version}) is outdated
-          Please install XQuartz #{latest_version}:
+          Your XQuartz (#{installed_version}) is outdated.
+          Please install XQuartz #{latest_version} (or delete it):
             https://xquartz.macosforge.org
         EOS
       end

Library/Homebrew/test/test_diagnostic.rb

@@ -85,7 +85,7 @@ class DiagnosticChecksTest < Homebrew::TestCase
 
   def test_check_homebrew_prefix
     # the integration tests are run in a special prefix
-    assert_match "Your Homebrew is not installed to /usr/local",
+    assert_match "Your Homebrew's prefix is not /usr/local.",
       @checks.check_homebrew_prefix
   end