Update prettier version to 1.13.6 (#1854)

Summary:
**Summary**

Updating to 1.13.6 which is the version used internally in Facebook's codebase, so that there are no conflicts in how prettier dictates formatting. Right now, we're seeing an issue where 1.13.6 dictates different formatting for nested ternary operators & this broke the Travis CI in 0cb80b7...fbc8a0c#diff-4e6b8232e09a07f699eebad1949b3ba3L118

Looks like the earlier breakage due to prettier version upgrade reported in #1562 is now not an issue with a much higher version :)

**Test Plan**
Before fix (breakage in Travis at https://travis-ci.org/facebook/draft-js/jobs/424170309):
```
$ eslint .
/home/travis/build/facebook/draft-js/src/model/immutable/BlockTree.js
  30:7  warning  'FINGERPRINT_DELIMITER' is assigned a value but never used. Allowed unused vars must match /^_/  no-unused-vars
/home/travis/build/facebook/draft-js/src/model/transaction/moveBlockInContentState.js
  119:33  error  Replace `⏎··········?·targetBlockIndex·-·1⏎·········` with `·?·targetBlockIndex·-·1`  prettier/prettier
✖ 2 problems (1 error, 1 warning)
  1 error, 0 warnings potentially fixable with the `--fix` option.
```

After fix:
```
> yarn run lint
Using globally installed version of Yarn
yarn run v1.5.1
$ eslint .

/Users/niveditc/draft-js/src/model/immutable/BlockTree.js
  30:7  warning  'FINGERPRINT_DELIMITER' is assigned a value but never used. Allowed unused vars must match /^_/  no-unused-vars

✖ 1 problem (0 errors, 1 warning)

✨  Done in 8.50s.
```
(will be fixing the warning separately, that's been ongoing for a while).
Pull Request resolved: #1854

Reviewed By: elboman

Differential Revision: D9636114

Pulled By: elboman

fbshipit-source-id: 037c692554976b4626e27d3ebc0ccf95922bfb38

package.json

@@ -73,7 +73,7 @@
     "gulp-uglify": "^1.2.0",
     "gulp-util": "^3.0.6",
     "jest": "^21.2.1",
-    "prettier": "1.9.1",
+    "prettier": "1.13.6",
     "react": "^16.0.0",
     "react-dom": "^16.0.0",
     "react-test-renderer": "^16.0.0",

yarn.lock

@@ -4485,9 +4485,9 @@ preserve@^0.2.0:
   version "0.2.0"
   resolved "https://registry.yarnpkg.com/preserve/-/preserve-0.2.0.tgz#815ed1f6ebc65926f865b310c0713bcb3315ce4b"
 
-prettier@1.9.1:
-  version "1.9.1"
-  resolved "https://registry.yarnpkg.com/prettier/-/prettier-1.9.1.tgz#41638a0d47c1efbd1b7d5a742aaa5548eab86d70"
+prettier@1.13.6:
+  version "1.13.6"
+  resolved "https://registry.yarnpkg.com/prettier/-/prettier-1.13.6.tgz#00ae0b777ad92f81a9e7a1df2f0470b6dab0cb44"
 
 pretty-format@^21.2.1:
   version "21.2.1"