debt - bring back opn dev dependency

package.json

@@ -116,6 +116,7 @@
     "mkdirp": "^0.5.0",
     "mocha": "^2.2.5",
     "mocha-junit-reporter": "^1.17.0",
+    "opn": "^6.0.0",
     "optimist": "0.3.5",
     "p-all": "^1.0.0",
     "pump": "^1.0.1",

yarn.lock

@@ -4493,6 +4493,11 @@ is-windows@^1.0.1, is-windows@^1.0.2:
   resolved "https://registry.yarnpkg.com/is-windows/-/is-windows-1.0.2.tgz#d1850eb9791ecd18e6182ce12a30f396634bb19d"
   integrity sha512-eXK1UInq2bPmjyX6e3VHIzMLobc4J94i4AWn+Hpq3OU5KkrRC96OAcR3PRJ/pGu6m8TRnBHP9dkXQVsT/COVIA==
 
+is-wsl@^1.1.0:
+  version "1.1.0"
+  resolved "https://registry.yarnpkg.com/is-wsl/-/is-wsl-1.1.0.tgz#1f16e4aa22b04d1336b66188a66af3c600c3a66d"
+  integrity sha1-HxbkqiKwTRM2tmGIpmrzxgDDpm0=
+
 is@^3.1.0, is@^3.2.1:
   version "3.2.1"
   resolved "https://registry.yarnpkg.com/is/-/is-3.2.1.tgz#d0ac2ad55eb7b0bec926a5266f6c662aaa83dca5"
@@ -5900,6 +5905,13 @@ oniguruma@^7.2.0:
   dependencies:
     nan "^2.14.0"
 
+opn@^6.0.0:
+  version "6.0.0"
+  resolved "https://registry.yarnpkg.com/opn/-/opn-6.0.0.tgz#3c5b0db676d5f97da1233d1ed42d182bc5a27d2d"
+  integrity sha512-I9PKfIZC+e4RXZ/qr1RhgyCnGgYX0UEIlXgWnCOVACIvFgaC9rz6Won7xbdhoHrd8IIhV7YEpHjreNUNkqCGkQ==
+  dependencies:
+    is-wsl "^1.1.0"
+
 optimist@0.3.5:
   version "0.3.5"
   resolved "https://registry.yarnpkg.com/optimist/-/optimist-0.3.5.tgz#03654b52417030312d109f39b159825b60309304"