git-annex: adjust assert for Ruby pre-1.8.7

Within a block, Ruby pre-1.8.7 gets confused about the parens in
this case.

Fixes mistydemeo/tigerbrew#194.

Formula/git-annex.rb

@@ -50,7 +50,7 @@ class GitAnnex < Formula
     # and propagate its content to another
     system "git", "clone", "my_annex", "my_annex_clone"
     Dir.chdir "my_annex_clone" do
-      assert (not File.file? "bigfile")
+      assert !File.file?("bigfile")
       system "git", "annex", "get", "bigfile"
       assert File.file? "bigfile"
     end