Update to swiglpk and GLPK v4.52
Updated .travis.yml and requirements to use swiglpk
Fixing optlang import errors during nosetests by python setup.py install
Refactored last occurences of glpk.glpkpi
Merge pull request #1 from biosustain/swiglpk

Move to swiglpk and GLPK 4.52