Use None, not False, for try_choose default value

[Manishearth]

False gets stored as 0

fixes #176

I also ran update table pull set try_choose=null where try_choose=0;
on the database to clean up things affected by this.

---

This change is 

[Manishearth]

r? @jdm

[jdm]

Fixing travis errors in #179.

[Manishearth]

@bors-servo try- r=jdm

[bors-servo]

📌 Commit f786798 has been approved by jdm

[bors-servo]

⌛ Testing commit f786798 with merge d2b0d20...

[bors-servo]

☀️ Test successful - status-travis
Approved by: jdm
Pushing d2b0d20 to master...

[Manishearth]

These PRs need re-approving after the sync

sqlite> select ("https://github.com/servo/"||repo||"/pull/"||num),repo,num,merge_sha from pull where merge_sha!="";
("https://github.com/servo/"||repo||"/pull/"||num)  repo        num         merge_sha
--------------------------------------------------  ----------  ----------  ----------------------------------------
https://github.com/servo/heapsize/pull/102          heapsize    102         d0da2c935673efadc0249297b7bc95314be00b74
https://github.com/servo/servo/pull/22159           servo       22159       3991151faabee07e4bb19fb24052009523564595
https://github.com/servo/servo/pull/22158           servo       22158       5b2c04aa6b2516d70eb504c4a03150b192141a92
https://github.com/servo/servo/pull/22150           servo       22150       72a10ff1f91cd025d8436c41f5b08d6c56158a45
https://github.com/servo/servo/pull/22149           servo       22149       66f75498df69c481cc6af5e3e6169bbfaeb66236
https://github.com/servo/servo/pull/22134           servo       22134       1e99a348607aa73e7023c61eedb049d024e236d0
https://github.com/servo/servo/pull/21926           servo       21926       8a11c28596294213c072130b0336279e151bad52
https://github.com/servo/servo/pull/21869           servo       21869       37644f02ed2e596cae48e902b19372583dfbe7aa
https://github.com/servo/servo/pull/21841           servo       21841       c0c86020eb00cba66bcb5bf64b2369feb5bac97a
https://github.com/servo/servo/pull/21801           servo       21801       3c6c7aa83db9bb9becdd3ecffe99a569345c3a20
https://github.com/servo/servo/pull/21780           servo       21780       6999271027d18f213357b71353bcf8c454192406
https://github.com/servo/servo/pull/21398           servo       21398       df86b74805a12f44800d068a5bbc1083a707e36f
https://github.com/servo/servo/pull/21370           servo       21370       d337e896f2b9d7f25cf85602f8219a46544687ea
https://github.com/servo/saltfs/pull/917            saltfs      917         67df6670cbd46e8399945fed2f93e048d2dee11f
https://github.com/servo/webrender/pull/3288        webrender   3288        2ac128bfc8b26f9213eb673ef6a366fc3077cccd
https://github.com/servo/heapsize/pull/101          heapsize    101         9c595ab355342669458322b234a53c5a06988fd2
https://github.com/servo/servo/pull/22147           servo       22147       c3956f8e236e11ad892c0ef92cf025028cd3d841
https://github.com/servo/osmesa-src/pull/31         osmesa-src  31          f0dc2b7c3bd323ef018e6845e0422baf02ef6d0a
https://github.com/servo/servo/pull/22091           servo       22091       8fac5b6ccbe7823feffbd39854f950604c8dc20e
https://github.com/servo/unicode-bidi/pull/50       unicode-bi  50          364ea5fbfbe72758f608f541d46b1a2513e3402d
https://github.com/servo/ipc-channel/pull/210       ipc-channe  210         62295b6a4f55bc493c8c00f2f59fb81d09d4f597

[Manishearth]

Oh looks like homu was smart enough to retry after the sync. Great!