Run LineEndings specs on Git 2.10 or newer

It turns on the `--eol` flag on `git ls-files` is only available on Git
2.10 or newer.

Until someone modifies the implementation to support older git versions,
skip the tests if running Git < 2.10.

Author: sds

lib/overcommit/hook/pre_commit/line_endings.rb

@@ -1,5 +1,7 @@
 module Overcommit::Hook::PreCommit
   # Checks for line endings in files.
+  #
+  # WARNING: Works with Git 2.10.0 or newer.
   class LineEndings < Base
     def run
       messages = []

spec/overcommit/hook/pre_commit/line_endings_spec.rb

@@ -12,12 +12,14 @@
       )
     )
   end
+
   let(:context) { double('context') }
   subject { described_class.new(config, context) }
   let(:eol) { "\n" }
   let(:staged_file) { 'filename.txt' }
 
   before do
+    skip('Skip LineEndings tests for Git < 2.10') if Overcommit::GIT_VERSION < '2.10'
     subject.stub(:applicable_files).and_return([staged_file])
   end