BLD: change sphinx-gallery dep to git

Author: jklymak

requirements/doc/doc-requirements.txt

@@ -13,6 +13,10 @@ ipython
 ipywidgets
 numpydoc>=0.8
 sphinxcontrib-svg2pdfconverter>=1.1.0
-sphinx-gallery>=0.7
+# sphinx-gallery>=0.7   
+# b41e328 is PR 808 which adds the image_srcset directive.  When this is 
+# released with sphinx gallery, we can change to the last release w/o this feature:
+# sphinx-gallery>0.90
+git+git://github.com/sphinx-gallery/sphinx-gallery@b41e328#egg=sphinx-gallery
 sphinx-copybutton
 scipy