restricting zoom in blockly. fix for #205
restricting zoom in blockly. fix for #205

switch-education/pxt,switchinnovations/pxt,Microsoft/pxt,Microsoft/pxt,switchinnovations/pxt,playi/pxt,switch-education/pxt,playi/pxt,Microsoft/pxt,playi/pxt,Microsoft/pxt,switch-education/pxt,switch-education/pxt,switch-education/pxt,switchinnovations/pxt,switchinnovations/pxt,Microsoft/pxt,playi/pxt