Remove 'problems' key (#468)

The 'problems' key is deprecated in favour of 'exercises' so we can
remove the unused 'problems'

config.json

@@ -4,90 +4,6 @@
   "repository": "https://github.com/exercism/xruby",
   "active": true,
   "gitter": "xruby",
-  "problems": [
-    "hello-world",
-    "hamming",
-    "gigasecond",
-    "rna-transcription",
-    "raindrops",
-    "difference-of-squares",
-    "pangram",
-    "sieve",
-    "roman-numerals",
-    "robot-name",
-    "nth-prime",
-    "leap",
-    "grains",
-    "word-count",
-    "bob",
-    "run-length-encoding",
-    "binary",
-    "accumulate",
-    "sum-of-multiples",
-    "grade-school",
-    "series",
-    "phone-number",
-    "prime-factors",
-    "strain",
-    "etl",
-    "trinary",
-    "beer-song",
-    "bowling",
-    "space-age",
-    "anagram",
-    "binary-search-tree",
-    "crypto-square",
-    "clock",
-    "alphametics",
-    "rail-fence-cipher",
-    "acronym",
-    "scrabble-score",
-    "nucleotide-count",
-    "flatten-array",
-    "hexadecimal",
-    "say",
-    "meetup",
-    "queen-attack",
-    "palindrome-products",
-    "bracket-push",
-    "food-chain",
-    "matrix",
-    "saddle-points",
-    "triangle",
-    "atbash-cipher",
-    "house",
-    "secret-handshake",
-    "proverb",
-    "ocr-numbers",
-    "pig-latin",
-    "simple-linked-list",
-    "luhn",
-    "simple-cipher",
-    "wordy",
-    "allergies",
-    "poker",
-    "kindergarten-garden",
-    "linked-list",
-    "pythagorean-triplet",
-    "robot-simulator",
-    "twelve-days",
-    "circular-buffer",
-    "largest-series-product",
-    "binary-search",
-    "two-bucket",
-    "pascals-triangle",
-    "custom-set",
-    "minesweeper",
-    "scale-generator",
-    "protein-translation",
-    "perfect-numbers",
-    "connect",
-    "list-ops",
-    "diamond",
-    "all-your-base",
-    "isogram",
-    "transpose"
-  ],
   "exercises": [
     {
       "slug": "hello-world",