RF: update .travis.yml to use pip for sympy
RF: update .travis.yml to use pip for sympy

alexis-roche/nipy,nipy/nireg,nipy/nireg,alexis-roche/niseg,arokem/nipy,nipy/nipy-labs,alexis-roche/register,alexis-roche/register,nipy/nipy-labs,alexis-roche/nipy,alexis-roche/register,bthirion/nipy,alexis-roche/nipy,alexis-roche/nireg,bthirion/nipy,alexis-roche/niseg,bthirion/nipy,bthirion/nipy,alexis-roche/nireg,arokem/nipy,arokem/nipy,arokem/nipy,alexis-roche/nipy