bugfix in conv2d_c01b.Conv.lmul()
added 'cython --version' to .travis.yml, to debug broken Travis installation procedure
force use of cython 0.20, as pytables doesn't like cython 0.21
Merge pull request #1133 from SuperElectric/conv2d_c01b_bugfix

bugfix in conv2d_c01b.Conv.lmul()