Switch to using ujson, remove simplejson from requirements

[iandees]

Related to tilezen/tilequeue#143 and tilezen/tilequeue#139.

Switch to using ujson, a faster JSON library, when loading the JSON tile to rebuild into a different format.

[nvkelso]

LGTM

[iandees]

I'm pretty sure we need to point to a new tagged version of tilequeue in this PR, right?

I was also working on setting up a profiling for tileserver.

[rmarianski]

I'm pretty sure we need to point to a new tagged version of tilequeue in this PR, right?

Wrt the requirements txt file checked into the repo? If so, I think it should just be left out.

[iandees]

The change on tileserver isn't nearly as impactful:

Coord	Format	Python 2.7.6 Built-in	ujson 1.3.5
1/1/1	json	290.2ms	300ms (3.4%)
1/1/1	topojson	233.4ms	213.8ms (-8.4%)
1/1/1	mvt	807.5ms	817.5ms (1.2%)
1/1/1	mvtb	814.9ms	873.9ms (7.2%)
7/32/47	json	715.9ms	689.7ms (-3.7%)
7/32/47	topojson	470.5ms	453.3ms (-3.7%)
7/32/47	mvt	1091.9ms	1055.4ms (-3.3%)
7/32/47	mvtb	1165.2ms	1115.3ms (-4.3%)
12/1050/1522	json	2207ms	2172.4ms (-1.6%)
12/1050/1522	topojson	1605.1ms	1654.1ms (3.1%)
12/1050/1522	mvt	3729.4ms	3667.7ms (-1.7%)
12/1050/1522	mvtb	3876ms	3515.9ms (-9.3%)
16/16812/24357	json	286.5ms	295.6ms (3.2%)
16/16812/24357	topojson	208.9ms	206.6ms (-1.1%)
16/16812/24357	mvt	559.1ms	500ms (-10.6%)
16/16812/24357	mvtb	523.3ms	571.7ms (9.2%)

[nvkelso]

Still looks overall better, so let's proceed with merging and testing on dev.