Avoid errors when git is not installed.

This will obviously not allow you to build the gem, but won't create issues
if you just want to rebuild the documentation and/or run the tests.

Signed-off-by: Aman Gupta <aman@tmm1.net>

Rakefile

@@ -82,7 +82,7 @@ Spec = Gem::Specification.new do |s|
   s.extra_rdoc_files  = Dir['README,docs/*']
 
   excludes            = %w(.gitignore)
-  s.files             = `git ls-files`.split("\n") - excludes
+  s.files             = `git ls-files`.split("\n") - excludes rescue Errno::ENOENT
 
   s.require_path      = 'lib'