Fix Pjs version

We use `Options.p` in a lot of places, but `.p` on Pjs classes wasn't
introduced until v3.1.0:
https://github.com/jneen/pjs/commit/0ce7b701d241fe7c0d8d12a172a2f353300adc7c

Thanks @robinwit!
https://mathquill.slack.com/archives/mathquill/p1461679514000080
https://mathquill.slack.com/archives/mathquill/p1461755912000101

package.json

diff --git a/package.json b/package.json
index e9889de..cedaec8 100644
--- a/package.json
+++ b/package.json
@@ -8,7 +8,7 @@
     "url": "https://github.com/mathquill/mathquill.git"
   },
   "dependencies": {
-    "pjs": "3.x"
+    "pjs": ">=3.1.0"
   },
   "devDependencies": {
     "mocha": "*",