feat: Update default Swift version to 4.1

Author: kylef

README.md

@@ -17,7 +17,7 @@ $ heroku create --buildpack https://github.com/kylef/heroku-buildpack-swift.git
 
 $ git push heroku master
 remote: -----> Swift app detected
-remote: -----> Installing Swift 4.0
+remote: -----> Installing Swift 4.1
 remote: -----> Installing clang-3.7.0
 remote: -----> Building Package
 remote: -----> Copying binaries to 'bin'
@@ -49,7 +49,7 @@ in your repository:
 
 ```shell
 $ cat .swift-version
-4.0
+4.1
 ```
 
 The `.swift-version` file is completely compatible with

bin/compile

@@ -11,7 +11,7 @@ BUILD_DIR=$1
 CACHE_DIR=$2
 ENV_DIR=$3
 
-DEFAULT_SWIFT_VERSION="4.0"
+DEFAULT_SWIFT_VERSION="4.1"
 CLANG_VERSION=3.7.0
 SWIFT_BUILD_CONFIGURATION="release"
 SWIFT_BUILD_FLAGS=""