Compute_20 nvcc error

Downloading and trying to install gives me the following error. I think this is because I'm using CUDA 9, which may have remove support for fermi architectures.

nvcc fatal : Unsupported gpu architecture 'compute_20'

This was simple enough to fix by making the following change:

```
diff --git a/build_sorting_libs.py b/build_sorting_libs.py
index 1589b7e..d1f44f6 100644
--- a/build_sorting_libs.py
+++ b/build_sorting_libs.py
@@ -53,7 +53,7 @@ def gencode_flags():

# Concatenate flags
SM = []
- SM.append(GENCODE_SM20)
+ # SM.append(GENCODE_SM20)
SM.append(GENCODE_SM30)
SM.append(GENCODE_SM35)
SM.append(GENCODE_SM37)
```

Start in build_sorting_libs.py, especially gencode_flags(), and inspect how the compute_20 flag is included in the generated nvcc arguments. Reproduce the installation with CUDA 9, then verify that the sorting libraries build without requesting compute_20 support.

Written by the indexing model from the issue text.