Don't depend on Foundation since Swift 4.2 (#58)

Before Swift 4.2 the `hasSuffix(_:)` method of `String` was unavailable on non-Darwin platforms without importing Foundation.
In Swift 4.2 this was fixed.

This allows us to not depend on Foundation on newer versions of Swift at all.

Sources/HTMLEntities/Constants.swift

@@ -14,8 +14,9 @@
  * limitations under the License.
  */
 
-// Linux toolchain requires Foundation to resolve `String` class's `hasSuffix()` function
-#if os(Linux)
+// Linux toolchain required Foundation to resolve `String` class's `hasSuffix()` function
+// before Swift 4.2.
+#if !swift(>=4.2) && os(Linux)
     import Foundation
 #endif