Error en jekyl - ubuntu 15.10

[dgomezcastro]

Recién instalado ruby y jekyll sin errores. Parece que _posts/en/2015-09-13-cohomologia_rham.md me da un error en jekyll al llamar a
jekyll server --trace
Al acceder a la página error: "Firefox can't establish a connection to the server at 127.0.0.1:4000."
A continuación la salida de terminal (actualización de git y llamada a jekyll con salida)

dgomez@dgomez-MS-7917:$ git clone https://github.com/mathifold/mathifold.git
Cloning into 'mathifold'...
remote: Counting objects: 1138, done.
remote: Compressing objects: 100% (158/158), done.
remote: Total 1138 (delta 53), reused 0 (delta 0), pack-reused 980
Receiving objects: 100% (1138/1138), 83.56 MiB | 1.52 MiB/s, done.
Resolving deltas: 100% (519/519), done.
Checking connectivity... done.
dgomez@dgomez-MS-7917:$ cd mathifold
dgomez@dgomez-MS-7917:/mathifold$ jekyll server --trace
Configuration file: /home/dgomez/mathifold/_config.yml
Source: /home/dgomez/mathifold
Destination: /home/dgomez/mathifold/_site
Generating...
Liquid Exception: undefined method extname' for #<Jekyll::StaticFile:0x000000025776a8> in _posts/en/2015-09-13-cohomologia_rham.md /home/dgomez/mathifold/_plugins/mod.rb:13:inblock in get_resources': undefined method extname' for #<Jekyll::StaticFile:0x000000025776a8> (NoMethodError) from /home/dgomez/mathifold/_plugins/mod.rb:9:ineach'
from /home/dgomez/mathifold/_plugins/mod.rb:9:in get_resources' from /home/dgomez/mathifold/_plugins/resource.rb:13:inget_file'
from /home/dgomez/mathifold/_plugins/resource.rb:68:in render' from /usr/lib/ruby/vendor_ruby/liquid/block.rb:151:inrender_token'
from /usr/lib/ruby/vendor_ruby/liquid/profiler/hooks.rb:5:in block in render_token_with_profiling' from /usr/lib/ruby/vendor_ruby/liquid/profiler.rb:80:inprofile_token_render'
from /usr/lib/ruby/vendor_ruby/liquid/profiler/hooks.rb:4:in render_token_with_profiling' from /usr/lib/ruby/vendor_ruby/liquid/block.rb:135:inblock in render_all'
from /usr/lib/ruby/vendor_ruby/liquid/block.rb:122:in each' from /usr/lib/ruby/vendor_ruby/liquid/block.rb:122:inrender_all'
from /usr/lib/ruby/vendor_ruby/liquid/block.rb:108:in render' from /usr/lib/ruby/vendor_ruby/liquid/template.rb:210:inblock in render'
from /usr/lib/ruby/vendor_ruby/liquid/template.rb:262:in with_profiling' from /usr/lib/ruby/vendor_ruby/liquid/template.rb:209:inrender'
from /usr/lib/ruby/vendor_ruby/liquid/template.rb:222:in render!' from /usr/lib/ruby/vendor_ruby/jekyll/convertible.rb:96:inrender_liquid'
from /usr/lib/ruby/vendor_ruby/jekyll/convertible.rb:211:in do_layout' from /usr/lib/ruby/vendor_ruby/jekyll/post.rb:268:inrender'
from /usr/lib/ruby/vendor_ruby/jekyll/site.rb:261:in block in render' from /usr/lib/ruby/vendor_ruby/jekyll/site.rb:260:ineach'
from /usr/lib/ruby/vendor_ruby/jekyll/site.rb:260:in render' from /usr/lib/ruby/vendor_ruby/jekyll/site.rb:43:inprocess'
from /usr/lib/ruby/vendor_ruby/jekyll/command.rb:53:in process_site' from /usr/lib/ruby/vendor_ruby/jekyll/commands/build.rb:50:inbuild'
from /usr/lib/ruby/vendor_ruby/jekyll/commands/build.rb:33:in process' from /usr/lib/ruby/vendor_ruby/jekyll/commands/serve.rb:24:inblock (2 levels) in init_with_program'
from /usr/lib/ruby/vendor_ruby/mercenary/command.rb:220:in call' from /usr/lib/ruby/vendor_ruby/mercenary/command.rb:220:inblock in execute'
from /usr/lib/ruby/vendor_ruby/mercenary/command.rb:220:in each' from /usr/lib/ruby/vendor_ruby/mercenary/command.rb:220:inexecute'
from /usr/lib/ruby/vendor_ruby/mercenary/program.rb:42:in go' from /usr/lib/ruby/vendor_ruby/mercenary.rb:19:inprogram'
from /usr/bin/jekyll:18:in `

'
dgomez@dgomez-MS-7917:/mathifold$

[s-nt-s]

Que te sale si escribes jekyll --version?
Debería darte jekyll 2.5.3