Fixed Derivative behaviour with t variable
Tighten Solve's check for tautologies (issue #208)

SymPy returns True if an equation is tautological. Since [1] == [True]
in Python, Mathics would accidentally treat an equation with one
result equal to 1 as a tautology. Fix this by using "is True" to check
for True.
Add test for issue #208
fix unevaluated index bug
lock django==1.6
Merge pull request #218 from sn6uv/travis_importlib

lock Djano at 1.6 (for this release)
Merge pull request #216 from bnjones/master

Tighten Solve's check for tautologies (issue #208)
Merge pull request #202 from GitAnt/DerivativeFix

Fixed Derivative behaviour with t variable
Merge pull request #217 from sn6uv/indexbug

fix unevaluated index bug
Update README.rst

update travis logos in readme
Added code to manage Sum[f[k],{k,Range[...]}]
Added test for Range in Sum
Removed rule from _IterationFunction (replaced by apply_range)