Reduce memory use of file_store.rb (#362)

## What

Writing to disk via `File.open` by default is buffered. By disabling the buffer via setting `sync = true` we can reduce memory allocation. It looks like no matter how small the amount of data being written is a default buffer size is used.

## Impact

While making a request to CodeTriage and measuring with memory_profiler via derailed benchmarks I see:

Before:
Total allocated: 742573 bytes (6644 objects)
     48741  rack-mini-profiler/lib

After:
Total allocated: 734236 bytes (6645 objects)
     40477  rack-mini-profiler/lib

This change accounts for 

Diff:

(742573 - 734236) / 742573.0 # => **1.1 % decrease in memory**

## Other Considerations

Is this slower than the buffered option?

```ruby
require 'benchmark/ips'

DATA = ["1ada37891cbb5442fa56157bc451a608", nil, 634602.016795]

def sync
  File.open("tmp-sync.dump", 'w') do |f|
    f.sync = true
    Marshal.dump(DATA, f)
  end
end

def no_sync
  File.open("tmp-no-sync.dump", 'w') do |f|
    Marshal.dump(DATA, f)
  end
end

Benchmark.ips do |x|
  x.report("no-sync") { no_sync }
  x.report("sync   ") { sync }
  x.compare!
end
# Warming up --------------------------------------
#              no-sync   597.000  i/100ms
#              sync      593.000  i/100ms
# Calculating -------------------------------------
#              no-sync      6.404k (±12.2%) i/s -     31.641k in   5.026470s
#              sync         6.416k (±12.4%) i/s -     32.022k in   5.079501s

# Comparison:
#              sync   :     6416.2 i/s
#              no-sync:     6404.3 i/s - same-ish: difference falls within error
```


It looks like the sync is not any slower and in most of my test runs was marginally faster (though well within standard deviation so it's impossible to make that claim in a statistically significant manner).

lib/mini_profiler/storage/file_store.rb

@@ -24,7 +24,10 @@ def [](key)
         end
 
         def []=(key,val)
-          ::File.open(path(key), "wb+") {|f| f.write Marshal.dump(val)}
+          ::File.open(path(key), "wb+") do |f|
+            f.sync = true
+            f.write Marshal.dump(val)
+          end
         end
 
         private