dont use concurrent requests on sapper export!!

i've spent so much time trying to figure out why sapper export often
misses exporting the [slug].json for certain pages. in b0b3b83 i
assumed the issue was (somehow) using hyphens in slugs so switched
to using underscores but this started happening again.

i found the followng issues which all are "closed" or "merged" but
this is _definitely_ still a bug. ended up looking at options for the
sapper export command and saw i can turn off concurrent requests,
which seems to have fixed the issue.
- sveltejs/sapper#1078
- sveltejs/sapper#893
- sveltejs/sapper#869

hoping to submit a repro of the issue to sapper soon

package.json

@@ -5,7 +5,7 @@
   "scripts": {
     "dev": "sapper dev",
     "build": "sapper build --legacy",
-    "export": "sapper export --legacy",
+    "export": "sapper export --legacy --concurrent 1",
     "start": "node __sapper__/build",
     "validate": "svelte-check --ignore src/node_modules/@sapper"
   },

pieces/avril-14th.yml

@@ -1,5 +1,5 @@
 title: Avril 14th
-slug: avril_14th
+slug: avril-14th
 date: 2015-11-03
 description: ""
 

pieces/songs-arent-time-travel.yml

@@ -1,5 +1,5 @@
 title: Songs Aren't Time Travel
-slug: songs_arent_time_travel
+slug: songs-arent-time-travel
 date: 2016-05-19
 description: ""