Add SourceKit response cache for reducing calls to SourceKit

This depends on jpsim/SourceKitten#135
By applying this, the duration of linting Carthage is reduced from:
```
swiftlint lint  27.85s user 1.98s system 77% cpu 38.538 total
```
to:
```
swiftlint lint  21.15s user 1.26s system 80% cpu 27.743 total
```

CHANGELOG.md

@@ -15,6 +15,9 @@
 * Add AutoCorrect for LegacyConstructorRule.  
   [Raphael Randschau](https://github.com/nicolai86)
 
+* Improve performance by reducing calls to SourceKit.  
+  [Norio Nomura](https://github.com/norio-nomura)
+
 ##### Bug Fixes
 
 * AutoCorrect for TrailingNewlineRule only removes at most one line.  

Source/SwiftLintFramework/Extensions/File+Cache.swift

@@ -8,8 +8,9 @@
 
 import SourceKittenFramework
 
-private var structureCache = Cache({file in Structure(file: file)})
-private var syntaxMapCache = Cache({file in SyntaxMap(file: file)})
+private var responseCache = Cache({file in Request.EditorOpen(file).send()})
+private var structureCache = Cache({file in Structure(sourceKitResponse: responseCache.get(file))})
+private var syntaxMapCache = Cache({file in SyntaxMap(sourceKitResponse: responseCache.get(file))})
 
 private struct Cache<T> {