Fix issue #5877 with tests

This pull request prevent to have lists environments inside the headings
cells which break latex code.

- First, the function `markdown2latex` has been changed in order to
  accept a markup keyword for pandoc reader, default to `markdown` as
  before(`test_markdown.py` updated too).
- Headings cells are treated with markup `markdown_strict+tex_math_dollars`
  to remove enhancements of pandoc for lists and add math support.
- Secondly, `prevent_list_blocks` filter escape characters `. - * +`
  in headings cells to prevent `enumerate` or `itemize` environments
  in latex sections(test added in `test_strings.py`

Test files accessible at:
    <https://gist.github.com/benjaminabel/4a40d1300dcf087c8695>
Build pandoc using haskell-platform in .travis.yml

This is required because travis use ubuntu 12.04 which ships with pandoc 1.9.11 this will not be required if travis update to ubuntu 14.04 whichuses version 1.12.2.1.
Get recent pandoc in travis from ppa cran2deb4ubuntu

This ppa:<https://launchpad.net/~marutter/+archive/ubuntu/c2d4u> contain
a backport of pandoc trusty package in precise. It can be used until
travis update his environment to ubuntu 14.04.
Merge pull request #6588 from benjaminabel/master

Handle numbers in heading cells when converting to Latex