chore: do not use lock file

See #1184 (comment)

.gitignore

@@ -1,3 +1,6 @@
+package-lock.json
+npm-shrinkwrap.json
+
 /coverage
 /docs/_build
 node_modules

.npmrc

@@ -1 +1,2 @@
 save-exact
+package-lock=false

docs/internals.rst

@@ -109,7 +109,7 @@ C++ dependencies
 
 Dredd uses `Drafter <https://github.com/apiaryio/drafter>`__ for parsing `API Blueprint`_ documents. Drafter is written in C++ and needs to be compiled during installation. Because that can cause a lot of problems in some environments, there’s also pure JavaScript version of the parser, `drafter.js <https://github.com/apiaryio/drafter.js>`__. Drafter.js is fully equivalent, but it can have slower performance. Therefore there’s `drafter-npm <https://github.com/apiaryio/drafter-npm/>`__ package, which tries to compile the C++ version of the parser and in case of failure it falls back to the JavaScript equivalent. Dredd depends on the `drafter-npm <https://github.com/apiaryio/drafter-npm/>`__ package.
 
-That still proved problematic for Dredd though. The current solution is to provide an `npm-shrinkwrap.json <https://docs.npmjs.com/files/shrinkwrap.json>`__ file with the `Dredd Transactions`_ library, which completely excludes `protagonist <https://github.com/apiaryio/protagonist>`__, i.e. the compiled C++ binding. Unlike ``package-lock.json``, the file can be distributed inside an npm package. The exclusion is performed by a ``postshrinkwrap`` npm script.
+That still proved problematic for Dredd though. The current solution is to provide an `npm-shrinkwrap.json <https://docs.npmjs.com/files/shrinkwrap.json>`__ file with the `Dredd Transactions`_ library, which completely excludes `protagonist <https://github.com/apiaryio/protagonist>`__, i.e. the compiled C++ binding. Unlike ``package-lock.json``, the file can be distributed inside an npm package. The exclusion is performed by a ``postshrinkwrap`` npm script. This didn't work well with Dredd's ``package-lock.json``, so currently Dredd's dependency tree is not locked for local or CI installations.
 
 
 Supported Node.js versions