chore(npm): remove lockfile

This removes our lockfile and our `ci-or-install` script. The lockfile
caused headaches, since `npm install` would modify the lockfile.

Unfortunately, `ci-or-install` (added in #896) didn't work reliably on
appveyor, so removing the lockfile seems easier. We remove the script
since it's now obsolete.

Fixes #893

.npmrc

@@ -0,0 +1,3 @@
+# We can't use lockfiles because npm@5 and npm@6 disagree about the syntax (and
+# will modify the lockfile when we run `npm install`).
+package-lock=false

.travis.yml

@@ -8,8 +8,6 @@ node_js:
   - 8
   - 9
 
-install:
-  - npm run ci-or-install
 os:
   - linux
   - osx

appveyor.yml

@@ -15,7 +15,7 @@ install:
   - set PATH=%APPDATA%\npm;%PATH%
   - node --version
   - npm --version
-  - npm run ci-or-install
+  - npm install
 
 matrix:
   fast_finish: false