[Haml] Don't kill rake gems:install if only the plugin skeleton is installed.

Closes gh-55

doc-src/HAML_CHANGELOG.md

@@ -27,6 +27,8 @@
   This is consistent with the behavior of multiple ids
   when one is specified as a standard attribute.
 
+* Don't crash if the plugin skeleton is installed and `rake gems:install` is run.
+
 ## [2.2.10](http://github.com/nex3/haml/commit/2.2.10)
 
 * Fixed a bug where elements with dynamic attributes and no content

doc-src/SASS_CHANGELOG.md

@@ -14,6 +14,8 @@
 
 * Fixed `css2sass`'s generation of pseudo-classes so that they're backslash-escaped.
 
+* Don't crash if the Haml plugin skeleton is installed and `rake gems:install` is run.
+
 ## [2.2.10](http://github.com/nex3/haml/commit/2.2.10)
 
 * Add support for attribute selectors with spaces around the `=`.

init.rb

@@ -1,8 +1,16 @@
 begin
   require File.join(File.dirname(__FILE__), 'lib', 'haml') # From here
 rescue LoadError
-  require 'haml' # From gem
+  begin
+    require 'haml' # From gem
+  rescue LoadError => e
+    # gems:install may be run to install Haml with the skeleton plugin
+    # but not the gem itself installed.
+    # Don't die if this is the case.
+    raise e unless defined?(Rake) && Rake.application.top_level_tasks.include?('gems:install')
+  end
 end
 
-# Load Haml and Sass
-Haml.init_rails(binding)
+# Load Haml and Sass.
+# Haml may be undefined if we're running gems:install.
+Haml.init_rails(binding) if defined?(Haml)