Downgrade nyc to ^11.0.0

nyc 12 requires Node 6.  On Node 4 it causes the following error:

    Transformation error for /home/travis/build/kevinoid/git-branch-is/bin/git-branch-is.js ; return original code
    Unexpected token ...
    Transformation error for /home/travis/build/kevinoid/git-branch-is/index.js ; return original code
    Unexpected token ...

Due to use of the spread operator in the instrumentation code, which
results in lack of instrumentation and incorrect coverage info (and a
test failure due to unexpected stderr output).

Stick to 11 as long as this package supports Node v4 and disable David
and Greenkeeper warnings.

Signed-off-by: Kevin Locke <[email protected]>

Author: kevinoid

package.json

@@ -32,7 +32,7 @@
     "lint-js": "eslint . && echo ESLint passed.",
     "postpublish": "git -C doc push && git push --follow-tags origin master gh-pages && echo Remember to update GitHub Releases from CHANGELOG.md && echo until skywinder/github-changelog-generator#56 is fixed.",
     "postversion": "rimraf doc && git clone -b gh-pages -l -q . doc && npm run doc && git -C doc add . && git -C doc commit -n -m \"Docs for v$npm_package_version\"",
-    "preversion": "depcheck --ignore-dirs doc && david && node ./bin/git-branch-is.js master && travis-status -b master -c -wx && appveyor-status -b master -c -w -p kevinoid/git-branch-is && nyc check-coverage --lines 95",
+    "preversion": "depcheck --ignore-dirs doc && david -i nyc && node ./bin/git-branch-is.js master && travis-status -b master -c -wx && appveyor-status -b master -c -w -p kevinoid/git-branch-is && nyc check-coverage --lines 95",
     "test": "npm run lint && npm run test-unit",
     "test-cov": "npm run lint && npm run test-unit-cov",
     "test-unit": "mocha --recursive test",
@@ -54,14 +54,19 @@
     "jsdoc": "^3.4.1",
     "mocha": "^5.0.0",
     "nodecat": "^1.0.0",
-    "nyc": "^12.0.0",
+    "nyc": "^11.0.0",
     "pify": "^3.0.0",
     "rimraf": "^2.2.0"
   },
   "engines": {
     "node": ">=4",
     "npm": ">=1.3.7"
   },
+  "greenkeeper": {
+    "ignore": [
+      "nyc"
+    ]
+  },
   "nyc": {
     "exclude": [
       "test",