Fix the build

Run `yarn install` in the js-sdkc and react-sdk installs. We were
avoiding this before due to using babel dependencies from riot-web
ro build js-sdk, which worked as long as the installed babel versions
and plugins were identical. js-sdk has now been upgraded to babel 7
and so that no longer worked.

This will probably cause react-sdk & js-sdk to be built an
unnecessary number of times.

package.json

@@ -31,6 +31,7 @@
     "i18n": "matrix-gen-i18n",
     "prunei18n": "matrix-prune-i18n",
     "diff-i18n": "cp src/i18n/strings/en_EN.json src/i18n/strings/en_EN_orig.json && yarn i18n && node scripts/compare-file.js src/i18n/strings/en_EN_orig.json src/i18n/strings/en_EN.json",
+    "postinstall": "node scripts/yarn-sub.js matrix-js-sdk install && node scripts/yarn-sub.js matrix-react-sdk install",
     "build:res": "node scripts/copy-res.js",
     "build:modernizr": "modernizr -c .modernizr.json -d src/vector/modernizr.js",
     "build:compile": "yarn reskindex && babel --source-maps -d lib src",