Set Swift version to 4.0, update Travis and pod spec.

.swift-version

@@ -1 +1 @@
-4.0-DEVELOPMENT-SNAPSHOT-2017-08-21-a
+4.0

.travis.yml

@@ -4,9 +4,9 @@ os:
 language: generic
 sudo: required
 dist: trusty
-osx_image: xcode8.3
+osx_image: xcode9
 install:
-  - eval "$(curl -sL https://gist.githubusercontent.com/kylef/5c0475ff02b7c7671d2a/raw/9f442512a46d7a2af7b850d65a7e9bd31edfb09b/swiftenv-install.sh)"
+  - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then eval "$(curl -sL https://gist.githubusercontent.com/kylef/5c0475ff02b7c7671d2a/raw/9f442512a46d7a2af7b850d65a7e9bd31edfb09b/swiftenv-install.sh)"; fi
 script:
 - swift build
 - swift test

SwiftShell.podspec

@@ -1,6 +1,6 @@
 Pod::Spec.new do |s|
   s.name         = 'SwiftShell'
-  s.version      = '3.0.0'
+  s.version      = '4.0.0'
   s.summary      = 'A Swift framework for shell scripting.'
   s.description  = 'SwiftShell is a library for creating command-line applications and running shell commands in Swift.'
   s.homepage     = 'https://github.com/kareman/SwiftShell'