hamming: JSON doesn't need to mention -1 anymore

[petertseng]

problem-specifications/exercises/hamming/canonical-data.json

Lines 9 to 10 in 2e46654

	"indicated with an expected value of -1. Note however that -1 is",
	"simply an indication here in the JSON. Actually returning -1 from",

Ever since #875 it's not necessary to mention -1 anymore.

Please:

• Change that text to mention the error object.
• Increment the last component of the version number:

  problem-specifications/exercises/hamming/canonical-data.json

  Line 3 in 2e46654

  "version": "2.1.0",

• Use a word in https://help.github.com/articles/closing-issues-using-keywords/ in the commit message.