first, confirm we can still get 0.7.5
attempt to have a sympy 0.7.5 and 0.7.6 matrix of tests
fix horz/vertcat test: should be error

but won't be on 0.7.5 (?) so xtest it
Merge pull request #149 from cbm755/ci_test_075_076

travis CI tests on both sympy 0.7.5 and 0.7.6, fix horz/vertcat xtests