Jpg optimization

There were a few unnecessary reads. Speed up is especially prominent on JRuby.

JPEG Image: 9,311,851 bytes
1,000,000 iterations

The following measurements are only for JPG initialization:
```ruby
Prawn::Images::JPG.new(img)
```

### MRI 1.9.3
```
             user     system      total        real    memory
Before: 32.770000   0.260000  33.030000 ( 33.491774)  34.5 MB
After:  19.600000   0.050000  19.650000 ( 19.793003)  29.6 MB
```
Speed: **1.68x faster**
Memory: **14% less**


### MRI 2.0.0
```
             user     system      total        real   memory
Before: 37.660000   0.200000  37.860000 ( 38.482895)  33.9 MB
After:  24.170000   0.060000  24.230000 ( 24.422936)  29.8 MB
```
Speed: **1.56x faster**
Memory: **12% less**


### JRuby 1.7.9
```
                user     system        total         real   memory
Before: 10487.800000 295.590000 10783.390000 (7131.061000)  528.3 MB (549.9 MB peak)
After:      7.350000   0.180000     7.530000 (   6.400000)  114.7 MB
```
Speed: **1,432x faster**
Memory: **78% less**


### Rubinius 2.2.1
```
             user     system      total        real   memory
Before: 21.270153   0.239070  21.509223 ( 20.862641)  150.1 Mb
After:  16.131227   0.186280  16.317507 ( 15.169135)  143.4 MB
```
Speed: **1.32x faster**
Memory: **4% less**