Update mpmath to 0.18-a1

The paths haven't been adapted yet.
Fix mpmath imports
Fix whitespace issues in mpmath
Use mpmath 0.18 final instead of 0.18-a1
Update mpmath docs
Fix mpmath index files
Fix mpmath docs
Fix the docs to build
Move Sage to the allowed failures in Travis

This is just until they update their version of mpmath. See the discussion at
https://github.com/sympy/sympy/pull/2784.
Merge pull request #2784 from asmeurer/mpmath-0.18

Update mpmath to 0.18