Lock down prettier to ~1.15.1 to prevent formatting changes in new minor versions from breaking other PRs

Author: papandreou

package.json

@@ -58,7 +58,7 @@
     "mocha-lcov-reporter": "^1.3.0",
     "nyc": "13.1.0",
     "offline-github-changelog": "1.2.0",
-    "prettier": "1.15.1",
+    "prettier": "~1.15.1",
     "sinon": "^7.1.0",
     "uglifyjs": "^2.4.11",
     "unexpected": "10.37.2",