Fixed future warning for pandas

PAtched issue from this thread jbkinney#36

logomaker/src/Logo.py

@@ -1070,7 +1070,7 @@ def _compute_glyphs(self):
         Specifies the placement and styling of all glyphs within the logo.
         """
         # Create a dataframe of glyphs
-        glyph_df = pd.DataFrame()
+        glyph_df = pd.DataFrame(index=self.ps,columns=self.cs,dtype=object)
 
         # For each position
         for p in self.ps: