Tell babel to use its cache.
Tell babel to use its cache.

This cuts initial compilation time in half for me.

Before:

```
webpack built 8ea813592373e2d5b5d8 in 12044ms
```

After:

```
webpack built 8ea813592373e2d5b5d8 in 6382ms
```

The incremental build doesn’t improve much.

desktop/desktop,shiftkey/desktop,hjobrien/desktop,hjobrien/desktop,kactus-io/kactus,desktop/desktop,shiftkey/desktop,BugTesterTest/desktops,say25/desktop,hjobrien/desktop,say25/desktop,shiftkey/desktop,kactus-io/kactus,BugTesterTest/desktops,j-f1/forked-desktop,desktop/desktop,say25/desktop,say25/desktop,kactus-io/kactus,j-f1/forked-desktop,j-f1/forked-desktop,kactus-io/kactus,BugTesterTest/desktops,gengjiawen/desktop,gengjiawen/desktop,hjobrien/desktop,gengjiawen/desktop,shiftkey/desktop,j-f1/forked-desktop,gengjiawen/desktop,BugTesterTest/desktops,artivilla/desktop,artivilla/desktop,desktop/desktop,artivilla/desktop,artivilla/desktop