Guo Xiang Tan 40ef5c4f14 PERF: Reduce memory allocation by Emoji.unicode_replacements.
Calling `Emoji.tonable_emojis` fetches from the cache but creates
a giant array each time it is called.

```
MemoryProfiler.report { Emoji.unicode_replacements }.pretty_print
```

Before:

```
Total allocated: 16560770 bytes (246364 objects)
Total retained:  784793 bytes (10624 objects)

allocated memory by gem
-----------------------------------
  10224581  activesupport-4.2.8
   3504241  hiredis-0.6.1
   1449466  discourse/app
    679314  2.4.1/lib
    419592  discourse/lib
    226720  redis-3.3.3
     56680  activerecord-4.2.8
       176  thread_safe-0.3.6
```

After:

```
Total allocated: 2127703 bytes (30556 objects)
Total retained:  798099 bytes (10735 objects)

allocated memory by gem
-----------------------------------
   1270762  discourse/app
    690149  2.4.1/lib
    106977  bootsnap-1.0.0
     48764  activesupport-4.2.8
      5266  logster-1.2.7
      2473  hiredis-0.6.1
      2056  redis-3.3.3
       840  discourse/lib
       240  activerecord-4.2.8
       176  thread_safe-0.3.6
```
2017-06-20 13:10:44 +09:00
..
2017-06-16 15:20:20 +04:30