Change latex output of Lambda to use \mapsto

A takeover of https://github.com/sympy/sympy/pull/2377.

@smichr Tests were failing because of parenthesis.

![first](https://cloud.githubusercontent.com/assets/2821646/3420758/95f980c8-fec0-11e3-9237-81f86e020e10.png)
is being produced as 
![second](https://cloud.githubusercontent.com/assets/2821646/3420757/5ef93e24-fec0-11e3-8dbb-52d57408e372.png)