Reduce benchmark volatility by disabling GC

Sparked by https://github.com/Shopify/liquid/pull/415#issuecomment-51926314, I took a look at why the Liquid benchmark had a standard deviation of 15-20%. The answer appears to be GC. I can now get the standard deviation consistently down below 5%, and often in the 2-3% range.

Three back to back runs on my machine:

```
parse:       58.4 (±1.7%) i/s -        588 in  10.068657s
parse:       58.2 (±1.7%) i/s -        585 in  10.060376s
parse:       57.3 (±3.5%) i/s -        575 in  10.047092s

parse & run:       25.3 (±3.9%) i/s -        254 in  10.042037s  
parse & run:       24.9 (±4.0%) i/s -        248 in  10.004997s
parse & run:       25.1 (±4.0%) i/s -        252 in  10.060830s
```

Output now looks a bit different due to trying to reduce memory usage and volatility between runs:

```
/usr/lib/shopify-ruby/2.1.2-shopify1/bin/ruby ./performance/benchmark.rb lax

Running parse: benchmark for 10 seconds (with 1 seconds warmup).

Calculating -------------------------------------
              parse:         5 i/100ms
-------------------------------------------------
              parse:       57.3 (±3.5%) i/s -        575 in  10.047092s

Running parse & run: benchmark for 10 seconds (with 1 seconds warmup).

Calculating -------------------------------------
        parse & run:         2 i/100ms
-------------------------------------------------
        parse & run:       25.1 (±4.0%) i/s -        252 in  10.060830s
```

@Shopify/liquid