Support travis cache builds (#39)

Author: lucas34

.travis.yml

@@ -1,6 +1,10 @@
 language: swift
 osx_image: xcode9
 
+cache:
+  directories:
+  - Carthage
+
 env:
   global:
   - LC_CTYPE=en_US.UTF-8
@@ -28,7 +32,7 @@ before_install:
 
 before_script:
   - swiftlint
-  - carthage bootstrap $PLATFORM
+  - carthage bootstrap $PLATFORM --cache-builds
 
 script:
   - danger process danger-swift