[conda] Make MXNet optional

conda/meta.yaml

@@ -43,7 +43,6 @@ requirements:
     - bob.pipelines
     - matplotlib {{ matplotlib }}
     - six {{ six }}
-    - mxnet
   run:
     - python
     - setuptools
@@ -53,6 +52,7 @@ requirements:
   optional:
     - bob.learn.tensorflow
     - tensorflow
+    - mxnet
 
 test:
   imports:

requirements.txt

@@ -20,5 +20,4 @@ bob.ip.facedetect
 bob.pipelines
 matplotlib   # for plotting
 six
-scikit-image
-mxnet
\ No newline at end of file
+scikit-image
\ No newline at end of file