inversekinematics: fix checking sympy_version to import correkt ikfast file
ikfast: merge version for sympy-0.7.4 into single ikfast.py file

Deleted ikfast_sympy0_7_4.py file on the way. Also reverted the related
check for sympy version from inversekinematics.py
ikfast.py: expand compatibility of new code down to sympy-0.7.3

It requires the same changes as for sympy-0.7.4, so ikfast should
run with both versions now.