Indicate use of stdin with 0.4.x compiler

[iamdefinitelyahuman]

What I did

Always include - to indicate use of stdin. This wasn't a requirement for compiling until 0.5.0, but it turns out it was always needed when combined with import_remappings.

Fixes #85

How I did it

In wrapper.py, remove the check for and solc_minor >= 5 when deciding if - should be added.

How to verify it

Run the tests. I've added a new case to verify the behavior of import_remappings when combined with compile_source.