fix: correct semantic-release peer dep version range

This should have been done with the change to the `generateNotes`
plugin format in abe1f7c. As of
this commit, this library supports `semantic-release` versions
from 15.7.0 until 15.9.x, before 15.10.0 changes the plugin system,
which will require a much more significant update.

Fixes #34

package.json

@@ -17,7 +17,7 @@
   "bin": "./bin/semantic-release-github-pr.js",
   "license": "MIT",
   "peerDependencies": {
-    "semantic-release": "13.4.1 - 15.6.x"
+    "semantic-release": "^15.7.0-15.9.x"
   },
   "dependencies": {
     "execa": "^0.8.0",
@@ -34,12 +34,12 @@
     "jest": "^22.0.4",
     "lint-staged": "^6.0.0",
     "prettier": "^1.8.2",
-    "semantic-release": "^15.0.0"
+    "semantic-release": "15.9.x"
   },
   "lint-staged": {
     "*.js": [
       "yarn format",
       "git add"
     ]
   }
-}
+}