Axis mappings may be strings instead of int

I got this traceback mailed to me:

Traceback (most recent call last):
  File "/Users/.../bin/fontmake", line 11, in <module>
    sys.exit(main())
  File "/Users/.../lib/python3.6/site-packages/fontmake/__main__.py", line 258, in main
    project.run_from_glyphs(glyphs_path, **args)
  File "/Users/.../lib/python3.6/site-packages/fontmake/font_project.py", line 560, in run_from_glyphs
    mti_source=mti_source)
  File "/Users/.../lib/python3.6/site-packages/fontTools/misc/loggingTools.py", line 375, in wrapper
    return func(*args, **kwds)
  File "/Users/.../lib/python3.6/site-packages/fontmake/font_project.py", line 131, in build_master_ufos
    font, family_name=family_name, instance_dir=instance_dir)
  File "/Users/.../lib/python3.6/site-packages/glyphsLib/builder/__init__.py", line 104, in to_designspace
    return builder.designspace
  File "/Users/.../lib/python3.6/site-packages/glyphsLib/builder/builders.py", line 211, in designspace
    self.to_designspace_axes()
  File "/Users/.../lib/python3.6/site-packages/glyphsLib/builder/axes.py", line 201, in to_designspace_axes
    minimum = min(mapping)
TypeError: '<' not supported between instances of 'str' and 'int'

@MrBrezina: can you please provide a test file? Maybe the one you used, stripped of all glyphs and miscellaneous data.

Start by inspecting glyphsLib/builder/axes.py around to_designspace_axes, where the traceback reports min(mapping) failing, and obtain the source .glyphs file requested in the issue. Reproduce the conversion with axis mappings containing strings and add coverage showing that the conversion completes with consistent integer mappings.