Add rake file to gemspec

Fixes #69

Thanks @antage

brakeman.gemspec

@@ -7,7 +7,7 @@ Gem::Specification.new do |s|
   s.summary = "Security vulnerability scanner for Ruby on Rails."
   s.description = "Brakeman detects security vulnerabilities in Ruby on Rails applications via static analysis."
   s.homepage = "http://brakemanscanner.org"
-  s.files = ["bin/brakeman", "WARNING_TYPES", "FEATURES", "README.md"] + Dir["lib/**/*.rb"] + Dir["lib/brakeman/format/*.css"]
+  s.files = ["bin/brakeman", "WARNING_TYPES", "FEATURES", "README.md", "lib/brakeman/brakeman.rake"] + Dir["lib/**/*.rb"] + Dir["lib/brakeman/format/*.css"]
   s.executables = ["brakeman"]
   s.add_dependency "activesupport"
   s.add_dependency "i18n"