Fix the check for git-svn

This seemed to cause an issue whenever there are no man pages installed
for git-svn.

Author: ignisf

ext/libv8/checkout.rb

@@ -44,7 +44,7 @@ def check_git_svn!
       # msysgit provides git svn
       return if RUBY_PLATFORM =~ /mingw/
 
-      unless system 'git help svn 2>&1 > /dev/null'
+      unless system 'git svn --version 2>&1 > /dev/null'
         fail "git-svn not installed!\nPlease install git-svn."
       end
     end