Fix failure on Travis because of telling tar to extract a gzip archive while it's already unpacked.

Apperantly OS X's BSD tar accepts the x option even if the archive is not a gzip archive.

lib/cocoapods/downloader/git.rb

@@ -82,7 +82,7 @@ module Pod
             tmpfile.write Zlib::GzipReader.new(archive).read
           end
           
-          system "tar zxf #{tmpfile.path} -C #{target_path} --strip-components 1"
+          system "tar xf #{tmpfile.path} -C #{target_path} --strip-components 1"
         end
         
         FileUtils.rm_f(tmp_path)