Minor correction concerning compilation compatibility with CUDA 9.0

Since CUDA 9.0 doesn't support sm_20 and sm_21 anymore. This PR allows Caffe to compile with CUDA 9.0 and newer versions successfully. 
This addresses the discussion concerning [#6237](BVLC/caffe#6237) in [master (#6237)](BVLC/caffe@cb150ec#diff-2004a3d3e6b4ed2e2812bb0b4b998f18)

Author: Coderx7

cmake/Cuda.cmake

@@ -113,8 +113,8 @@ function(caffe_select_nvcc_arch_flags out_variable)
   set(__nvcc_flags "")
   set(__nvcc_archs_readable "")
 
-  string(COMPARE GREATER_EQUAL "${CUDA_VERSION}" "9.1" iscudanewerthan91)
-  if(iscudanewerthan91)
+  string(COMPARE GREATER_EQUAL "${CUDA_VERSION}" "9.0" iscudanewerthan90)
+  if(iscudanewerthan90)
     string(REPLACE "21(20)" "" __cuda_arch_bin "${__cuda_arch_bin}")
     string(REPLACE "20" "" __cuda_arch_bin "${__cuda_arch_bin}")
   endif()