Add string-to-array-buffer as dev dependency

Used for testing attachment data migration.

package.json

@@ -111,6 +111,7 @@
     "nyc": "^11.4.1",
     "sinon": "^4.4.2",
     "spectron": "^3.8.0",
+    "string-to-arraybuffer": "^1.0.0",
     "tempy": "^0.2.1",
     "tmp": "^0.0.33"
   },

yarn.lock

@@ -340,6 +340,10 @@ asynckit@^0.4.0:
   version "0.4.0"
   resolved "https://registry.yarnpkg.com/asynckit/-/asynckit-0.4.0.tgz#c79ed97f7f34cb8f2ba1bc9790bcc366474b4b79"
 
+atob-lite@^2.0.0:
+  version "2.0.0"
+  resolved "https://registry.yarnpkg.com/atob-lite/-/atob-lite-2.0.0.tgz#0fef5ad46f1bd7a8502c65727f0367d5ee43d696"
+
 atob@~1.1.0:
   version "1.1.3"
   resolved "https://registry.yarnpkg.com/atob/-/atob-1.1.3.tgz#95f13629b12c3a51a5d215abdce2aa9f32f80773"
@@ -2915,6 +2919,10 @@ is-arrayish@^0.2.1:
   version "0.2.1"
   resolved "https://registry.yarnpkg.com/is-arrayish/-/is-arrayish-0.2.1.tgz#77c99840527aa8ecb1a8ba697b80645a7a926a9d"
 
+is-base64@^0.0.4:
+  version "0.0.4"
+  resolved "https://registry.yarnpkg.com/is-base64/-/is-base64-0.0.4.tgz#6ef33c01f4870d1306ea38857bd59adf3e3f543f"
+
 is-buffer@^1.1.5:
   version "1.1.6"
   resolved "https://registry.yarnpkg.com/is-buffer/-/is-buffer-1.1.6.tgz#efaa2ea9daa0d7ab2ea13a97b2b8ad51fefbe8be"
@@ -5151,6 +5159,13 @@ string-similarity@1.1.0:
   dependencies:
     lodash "^4.13.1"
 
+string-to-arraybuffer@^1.0.0:
+  version "1.0.0"
+  resolved "https://registry.yarnpkg.com/string-to-arraybuffer/-/string-to-arraybuffer-1.0.0.tgz#6aeadd45b58e633c9e833c7ee6626ee6950dd3c5"
+  dependencies:
+    atob-lite "^2.0.0"
+    is-base64 "^0.0.4"
+
 string-width@^1.0.1, string-width@^1.0.2:
   version "1.0.2"
   resolved "https://registry.yarnpkg.com/string-width/-/string-width-1.0.2.tgz#118bdf5b8cdc51a2a7e70d211e07e2b0b9b107d3"