Swift 4.0 compatibility (#1)

* made swift 4.0 compatible

* added Swift 4.0 to .travis.yml

* added backport of compactMap for swift 4.0

* changed extension to Array

* handling swift version inline instead of trying to backport.

.travis.yml

@@ -15,6 +15,7 @@ os:
   - osx
 
 env:
+  -  SWIFT_VERSION=4.0
   -  SWIFT_VERSION=4.1
 
 before_install:

Sources/parsers/MultiFlag.swift

@@ -30,15 +30,25 @@ public class MultiFlag: Parser, ParserNode, ParsePathSegment {
                 prefixString + String($0)
             })
             // find a valid flag parser for each flag
-            let matched: [(parser: Flag, flag: String)] =
-                flags.compactMap({ flag in
-                    if let parser = parsers
-                        .compactMap({ $0 as? Flag })
-                        .first(where: { parser in parser.aliases.contains(flag) }) {
-                        return (parser: parser, flag: flag)
-                    }
-                    return nil
-                })
+#if swift(>=4.1)
+            let matched: [(parser: Flag, flag: String)] = flags.compactMap({ flag in
+                if let parser = self.parsers
+                    .compactMap({ $0 as? Flag })
+                    .first(where: { parser in parser.aliases.contains(flag) }) {
+                    return (parser: parser, flag: flag)
+                }
+                return nil
+            })
+#else
+            let matched: [(parser: Flag, flag: String)] = flags.flatMap({ flag in
+                if let parser = self.parsers
+                    .flatMap({ $0 as? Flag })
+                    .first(where: { parser in parser.aliases.contains(flag) }) {
+                    return (parser: parser, flag: flag)
+            }
+                return nil
+            })
+#endif
             // check if all flags are valid flags, do not treat this as multi flag otherwise
             if matched.count == flags.count {
                 try matched.forEach { parser, flag in