Update travis.yml to retry package installations when they fail
Attempt to fix travis.yml
Add object iterator for environment objects

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Add options

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Move sexpr to util directory

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Add initializer list constructor for hierarchical names

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Add pretty printer for s-expressions and options

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>