Update readme and .travis.yml to add swift 4.

.travis.yml

@@ -86,6 +86,8 @@ matrix:
     - {env: IMPL=swift NO_DOCKER=1, os: osx, osx_image: xcode7.3}
     - {env: IMPL=swift3,    services: [docker]}
     - {env: IMPL=swift3 NO_DOCKER=1, os: osx, osx_image: xcode8}
+    - {env: IMPL=swift4,    services: [docker]}
+    - {env: IMPL=swift4 NO_DOCKER=1, os: osx, osx_image: xcode10}
     - {env: IMPL=tcl,       services: [docker]}
     - {env: IMPL=ts,        services: [docker]}
     - {env: IMPL=vb,        services: [docker]}

README.md

@@ -6,7 +6,7 @@
 
 **1. Mal is a Clojure inspired Lisp interpreter**
 
-**2. Mal is implemented in 74 languages**
+**2. Mal is implemented in 75 languages**
 
 | Language | Creator |
 | -------- | ------- |
@@ -77,6 +77,7 @@
 | [Skew](#skew) | [Dov Murik](https://github.com/dubek) |
 | [Swift](#swift) | [Keith Rollin](https://github.com/keith-rollin) |
 | [Swift 3](#swift-3) | [Joel Martin](https://github.com/kanaka)  |
+| [Swift 4](#swift-4) | [陆遥](https://github.com/LispLY)  |
 | [Tcl](#tcl-86) | [Dov Murik](https://github.com/dubek) |
 | [TypeScript](#typescript) | [Masahiro Wakame](https://github.com/vvakame) |
 | [VHDL](#vhdl) | [Dov Murik](https://github.com/dubek) |
@@ -930,6 +931,17 @@ make
 ./stepX_YYY
 ```
 
+### Swift 4
+
+The Swift 4 implementation of mal requires the Swift 4.0 compiler. It
+has been tested with Swift 4.2.3 release.
+
+```
+cd swift4
+make
+./stepX_YYY
+```
+
 ### Tcl 8.6
 
 The Tcl implementation of mal requires Tcl 8.6 to run.  For readline line