Revert "Update README with correct dependencies. (#772)"

This reverts commit db1a8a3.

We should keep the dependency versions listed in the README consistent
with the `ndarray` version listed in the same README.

Author: jturner314

README.rst

@@ -81,8 +81,8 @@ provider::
 
     [dependencies]
     ndarray = { version = "0.13.0", features = ["blas"] }
-    blas-src = { version = "0.4.0", default-features = false, features = ["openblas"] }
-    openblas-src = { version = "0.7.0", default-features = false, features = ["cblas", "system"] }
+    blas-src = { version = "0.2.0", default-features = false, features = ["openblas"] }
+    openblas-src = { version = "0.6.0", default-features = false, features = ["cblas", "system"] }
 
 For official releases of ``ndarray``, the versions are: