Verify exercise difficulties (#503)

* Adjust config.json to return exercise difficulties to one of 1, 4, 7, 10.

Performed with the following command line:

```shell
jq '.exercises |= map(
    if .difficulty == 3 then
       .difficulty=4
    elif .difficulty == 6 then
       .difficulty=7
    else
        .
    end
)' config.json > config.json.new && \
diff config.json config.json.new ; \
mv config.json.new config.json
```

* Add script to verify exercise difficulties

Encompasses #502 and should probably be merged afterward.

* Specify correct path for verify-exercise-difficulties

* Add executable bit to _test/*.sh

Author: coriolinus

.travis.yml

@@ -8,6 +8,7 @@ script:
 - "./bin/fetch-configlet"
 - "sh ./_test/ensure-readmes-are-updated.sh"
 - "./bin/configlet lint ."
+- "./_test/verify-exercise-difficulties.sh"
 sudo: false
 rust:
   - stable

_test/check-stubs.sh

@@ -0,0 +1,52 @@
+#!/bin/bash
+
+repo=$(cd "$(dirname "$0")/.." && pwd)
+config=$repo/config.json
+
+es=0
+
+# ensure every exercise has a difficulty
+no_difficulty=$(
+    jq --raw-output '
+        .exercises[] |
+        select((.deprecated | not) and (has("difficulty") | not)) |
+        .slug
+    ' $config
+)
+if [ -n "$no_difficulty" ]; then
+    echo "Exercises without a difficulty in config.json:"
+    echo "$no_difficulty"
+    es=1
+fi
+
+# ensure that all difficulties are one of 1, 4, 7, 10
+invalid_difficulty=$(
+    jq --raw-output '
+        .exercises[] |
+        select(
+            (.deprecated | not) and
+            has("difficulty") and
+            (
+                .difficulty | tostring |
+                in({"1":null,"4":null,"7":null,"10":null}) |
+                not
+            )
+        ) |
+        "\(.slug) (\(.difficulty))"
+    ' $config
+)
+if [ -n "$invalid_difficulty" ]; then
+    echo "Exercises with invalid difficulty (must be in {1, 4, 7, 10})"
+    echo "$invalid_difficulty"
+    es=1
+fi
+
+# ensure difficulties are sorted
+exercise_order=$(jq --raw-output '.exercises[] | select(.deprecated | not) | .slug' $config)
+sorted_order=$(jq --raw-output '.exercises | sort_by(.difficulty) | .[] | select(.deprecated | not) | .slug' $config)
+if [ "$exercise_order" != "$sorted_order" ]; then
+    echo "Exercises are not in sorted order in config.json"
+    es=1
+fi
+
+exit $es