Add rxjs to devDependencies

This is needed in development because yarn doesn't install
peerDependencies by default.

Yarn issue: yarnpkg/yarn#1503

package.json

@@ -49,6 +49,7 @@
     "cpy-cli": "^2.0.0",
     "mkdirp": "^0.5.1",
     "mocha": "^5.2.0",
+    "rxjs": "^6.4.0",
     "rimraf": "^2.6.3",
     "sinon": "^6.0.0",
     "sinon-chai": "^3.2.0",

yarn.lock

@@ -2926,6 +2926,13 @@ run-queue@^1.0.0, run-queue@^1.0.3:
   dependencies:
     aproba "^1.1.1"
 
+rxjs@^6.4.0:
+  version "6.5.4"
+  resolved "https://registry.yarnpkg.com/rxjs/-/rxjs-6.5.4.tgz#e0777fe0d184cec7872df147f303572d414e211c"
+  integrity sha512-naMQXcgEo3csAEGvw/NydRA0fuS2nDZJiw1YUWFKU7aPPAPGZEsD4Iimit96qwCieH6y614MCLYwdkrWx7z/7Q==
+  dependencies:
+    tslib "^1.9.0"
+
 safe-buffer@^5.0.1, safe-buffer@^5.1.0, safe-buffer@^5.1.1, safe-buffer@^5.1.2, safe-buffer@~5.2.0:
   version "5.2.0"
   resolved "https://registry.yarnpkg.com/safe-buffer/-/safe-buffer-5.2.0.tgz#b74daec49b1148f88c64b68d49b1e815c1f2f519"