Change testing requirement to tensorflow-gpu

setup.cfg

@@ -43,7 +43,7 @@ test_requires =
     pytest-html
     ansi2html
     pytest-cov
-    tensorflow
+    tensorflow-gpu
 # Require a specific Python version, e.g. Python 2.7 or >= 3.4
 # python_requires = >=2.7,!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*
 
@@ -62,7 +62,7 @@ testing =
     pytest-html
     ansi2html
     pytest-cov
-    tensorflow
+    tensorflow-gpu
 pybind11 =
     cppimport