Doubly specified packages are ignored

[andy-maier]

In requirements files, sometimes one needs different versions depending on environment markers such as the Python version. For example:

httpretty>=0.8.14,<0.9.1; python_version == '2.6'
httpretty>=0.9.5; python_version > '2.6'

What happens is that pyup essentially ignores the package entirely. In the pyup dashboard for the project, such packages do not show up at all.

I am aware of issue #279, but I am opening this new issue to point out that the package is ignored.

[rafaelpivato]

Another requirements parsing limitation as mentioned on #345