Update .travis.yml

- force to use precompiled cmake
- do not delete old doxygen files (will fix later)
[skip ci]
Make unification_problems a virtual class. Associate a 'standard' context with each metavariable in metavar_env

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Make unification_problems a virtual class. Associate a 'standard' context with each metavariable in metavar_env